Hi Kristin … and Koala,
I agree with both of you on this 
One of the most frustrating things that I consistently encounter in my research is this very issue where there are many different and conflicting opinions about almost every issue that concerns mattresses … and all of them can often come from people I respect, have been in the industry for years, and would seem to know. I can’t tell you how many times I have spent many many hours trying to resolve conflicting information surrounding many different theories and finding a resolution to these types of issues. This is where intuition and science need to be blended and where both left and right brained thinking by themselves can end up being misleading.
There are several reasons for these conflicts IMO (at least the ones where everyone is “right” in certain circumstances because there are some where which opinion is more likely to be “wrong” or right" is much more clear).
First are the ideas about mattress construction and how they interact with people. There is an infinite degree of differences between people both in terms of height and weight distribution, body shape, minor differences in “habitual” sleeping positions and patterns which can make a big difference, and also differences between tolerances to either pressure or misalignment between different people who otherwise may be very similar. We are all made very differently and have very different perceptions and preferences which can also have a great effect on how well we sleep. Some of these differences can be actual physiological differences such as different thicknesses of insulating fat under the skin which can affect the pressure on nerves or restriction of blood vessels or differences in joint flexibility or long term chronic underlying conditions or influences which people may or may not even be aware of. These types of differences can lead to a very wide variety of differences in mattress construction and layering that may work “best” for different people who seem very similar. All of this is part of both the “challenge” and “rewards” of mattress theory and construction. Most people fall inside a range which can accommodate them but for those who are outside of the typical range that is the “norm”, it can become very frustrating when they see so many others just like them who are happy with a mattress that just doesn’t work for them.
Second though is the differences in risk tolerance between different people … either manufacturers or consumers. A solid surface platform vs a slatted base foundation is an example of this. It’s true that latex does breathe more than other foams and that natural latex also has an inherent resistance to mold and mildew and would have less likelihood of developing mold and mildew for these reasons. It’s also true though that the development of mold and mildew would depend on a combination of several factors coming together. One of the most important of these is moisture (from the environment or the person on the mattress), one is the types of mold or mildew spores that are in the environment, and one is a food source (cellulose is one of these). For example the temperature difference between a mattress and a solid surface foundation can play a role in condensation which would increase the odds that mold or mildew could develop. A cover or better yet an insulated cover on a foundation could help with this temperature differential vs just having a mattress on wood.
Environmental conditions and degrees of humidity could also play a role. The degree of moisture in the wood itself would also be one of the variables. The fabric that was used in the mattress ticking and the quilting could also increase or decrease the likelihood of moisture retention and mold. A bedroom in a basement would be more likely to have issues than one on the third floor. A mattress on a floor would also be at greater risk than one that was higher up in the air which was warmer and could come to room temperature more quickly, even in the same room. The degree of moisture that each person releases over the course of the night would play a role. There are many more factors that could be involved but the idea I’m hoping to get across is that there is usually a combination of factors that leads to mold or mildew or many other “issues”. Mold and mildew is also not the only issue involved because the airflow through a mattress can also affect the dust mite population (which depends on humidity in their surroundings to absorb the moisture that they need to survive) and can also affect the durability of a mattress (higher humidity can reduce the useful life of many mattress materials).
The question then is not so much about who is “right” or “wrong” which are opposite ends of a spectrum but trying to assess where in the spectrum you may fall and how comfortable you are with being there. I personally believe that a slatted foundation moves the “risk” towards the safer side of the spectrum and if the slats are strong and firm enough and close enough together there is no real down side to using them. This doesn’t mean that a sheet of plywood will be a problem in most circumstances … only that the odds that it could are increased … both because the plywood can absorb moisture over time and because it reduces the ability for moisture to evaporate compared to slats. It would be a bigger problem with less breathable materials like many synthetic fabrics or less breathable foams and natural latex is also inherently “resistant” to mold to some degree but they all have a cover and IMO the risk is still there with latex as well although to a reduced degree.
There are also other considerations that may be important to some people as well such as the glue that is used in plywood (offgassing) and the benefits of greater breathability in terms of small improvements in sleeping microclimate. The bottom of a mattress is a significant part of it’s surface area and when you move and “push” air through the mattress … reducing the ability of the bottom of the mattress to allow airflow can reduce to some degree the breathability of the entire mattress although this is not usually a major issue. Innersprings for example are in general more “breathable” than foam mattresses for this very reason because they allow more airflow than any foam even though they are deeper inside the mattress.
In other cases it may just be a choice that someone makes because of a tradeoff such as the purchase of an adjustable bed which has a solid surface and the benefits of the adjustable bed far outweighs any additional risk that may be connected with its solid surface (I use an adjustable bed myself).
So in the end … the resolution to all of these “conflicts” is a combination of intuition, fact, and tradeoffs, and recognizing that both “wrong” and “right” are equally unlikely to be “accurate”.
I personally believe that a slatted or wire grid foundation is a better idea than plywood or some other solid surface (such as hard plastic or cardboard) unless there is a compelling reason to choose otherwise but this is only because it is such a simple way to reduce the risk (which involves more than just cosmetics and the appearance of the mattress), has other benefits, and there is no real advantage that I know of to a solid surface vs a strong, non flexing, and evenly supportive slatted surface.
For those that have a solid surface support system under their mattress then something like a this bed rug in between the mattress and the foundation or one of the slat conversions here (which has no flex at all) or even one of the Ikea slatted bed bases here (which has some flex which may change how the mattress feels and performs) could work well and would also increase ventilation under their mattress.
While mold issues start long before they are visible … there’s at least one person who had an issue (there’s actually more that a web search will bring up) with natural Dunlop latex although this was likely a combination of “moisture producing” factors.
http://www.diaperswappers.com/forum/archive/index.php/t-674120.html
Phoenix