After some experimentation, I am leaning to the conclusion that to produce a good spin, a racquet must be powerful. Powerful is not the same a stiff. I've seen plenty of racquets which are stiff and dead at the same time ( meaning no power ). Soft also does not mean power-less. Head PT630 and Volkl C10 are very soft, and yet also powerful. I am not sure what it is that makes the racquet return the power back to the ball, but whatever it is, you need a lot of it. Otherwise the racquet head speed needs to increase ~4X to get you the same trajectory you had before. This much acceleration increase may be too much of a jump, too radical for recreational players.
Its either this, or your racquet power needs to increase 4X. If you go with a new powerful racquet, you'll need to slow down your existing flat shots about 4 times to account for the new extra power. Or, if you stick with your current racquet, you'll need to produce violent bursts of head speed to produce some proper spin.
Neither of these adjustments are particularly easy to master, let alone mix successfully into one game. This is probably why for those adventurous people who want to diversify their game, it ends up just "a little bit of extra spin" here, a little bit of "flat" there. And this is probably why the majority of people play one style only -- either flat all the time, or spin all the time.