Hi FirmSleeper,
There are a few other manufacturers that insist on using their own foundations as part of their warranty criteria (including Tempurpedic) but it’s not the norm. Most manufacturers have criteria that include a “suitable” foundation but it doesn’t have to be their own.
I think it’s unfortunate when a manufacturer does this because while they are certainly high quality foundations … I think that the price they are charging (like their mattresses) is higher than it “should be” when there are many other similar quality foundations that would also be similar in quality and would provide suitable support for their mattresses that are in lower price ranges (such as some of the ones that are listed in the foundation topic here).
Even though it’s unlikely that you would need a warranty when the materials in your mattress are very high quality … there is always the possibility that you receive a defective layer and I don’t think it would be “worth” buying a mattress without one.
As you know I think very highly of Flobeds and they are a member of this site and they have some great quality mattresses available and some unique zoning options as well.
Phoenix