Unless you have a great onboard air compressor or an air tank, having a deflated spare would do you no good. You couldn't deflate it under 15 or so reliably without risking the bead seal, and you don't want to be sitting on the side of the road for an extra 15 minutes airing up a tire. And I don't think putting it down to 15 psi would make is small enough to fit anyway.
EDIT: I always carry onboard air and I still wouldn't want to do this. Imagine being late for something and having a flat. With the spare aired up I could have it changed in 10 minutes easily. Add another 5-10 to that and that's still 5-10 more minutes that you're late. And on the trail, it's not cool to make people wait for you to air up a spare.