I went ahead and did it. It looks like most of the benefits are fluff, but if I use three guest certificates (which is likely) or extra vacations (not likely) in the next two years, I'll come out ahead.
The ShortStays look like they might be beneficial for us, too. If they do, we'll get two exchanges out of a deposit and save $15-$45 on the exchange fees. It's fairly common that we'll make exchanges and use less than the 7 full days. With cheap deposits and low exchange fees, it's often cheaper than 3-4 nights in a hotel (and sometimes even 2). Not seeing a tremendous selection there, but as much as we travel I'm sure we'll find one here and there.