How to use fruit purees instead of fats in baked goods

I've always found this interesting, the idea that you can use apple sauce instead of oil or butter and everything will turn out great. Seems suspect. Not that I haven't done it before; I used apple sauce in place of oil in sugar cookies once. They weren't bad per se, they just weren't that good. And now I know why. I was listening to public radio on the way to my parents' house and the Splendid Table was on. I remember when this show came out, but I never got to listen to it because it's on Sunday mornings and I was being dragged to church; more recently I just forgot about it. But I found myself in the car on Sunday, and Lynne and I were cruising south. Anyway, a woman came on with a question about subbing apple sauce or other fruit/carrot purees in baked goods, and here's what we both found out:

• You should NOT sub purees for fat in recipes that use the creaming method. In other words, most cakes and cookies ("cream butter and sugars..."). These baked goods get their fluff from the sugar cutting through the fat and creating air pockets. No fat, no fluff, end of story.
  
• You should NOT sub purees for fat in recipes that use beaten eggs as the fluffing agent. I was thinking angel food cake...but that doesn't have any fat anyway...so, you know, other things with beaten eggs.
• You CAN sub purees for fat in recipes that basically don't fall under the other 2 categories. Anything where you can dump all the ingredients. Things like carrot cake and hummingbird cake are perfect candidates (I'm debating box cake mixes...maybe). Lynne was most excited about the possibility of brownies, especially using a prune puree. (brown-laxies. yum!) Seriously though, that sounds delicious. She said it works great with anything chocolate.
• On a final note Lynne (don't you love that we're on a first name basis) said that you shouldn't sub puree for all the fat, because, well gosh darn it (channeling Alton Brown), it tastes good. (and it serves other functions...)

So, let's review. No purees in cookies or anything else using the creaming method. No purees in egg-induced fluffiness. Go puree-crazy in brownies and 'dump' recipes. But don't leave out all the fat.