Hi Strummer,
Like you … I think that these “insurance” plans are a real racket and are almost all pure profit. If you do have to make a claim on them … they will usually just send you out a cleaning kit. I would never buy these.
A mattress protector on the other hand is more important IMO and they can also make a difference in how your mattress feels and in temperature control as well. The thinner membrane type (like the protect-a-bed) can be good choices and are fairly inexpensive (although as you pointed out they can vary widely) but they are also less breathable than some other types (such as some of those listed in post #89 here) so they can be a factor in the sleeping temperature of your mattress (in combination with the type of foam used in the comfort layers, the ticking and quilting of the mattress, the protector, and the sheets and bedding you use).
If you found one that was good quality (preferably natural fibers or a viscose type of fiber) for 19.99 I certainly wouldn’t hesitate to buy it 
Phoenix