I admit that I didn't read this entire thread, but I'll plop down my own thoughts (it's the *in* thing to do, right?).
Last I checked, I probably have 800 megabytes of downloaded .pdfs in my Bearshare folder. Which, is probably in actuality a lot less than you might think it is, since a big WoTC book can be about 75 megs. I've got probably 40 files or so. And, I also keep my BearShare folder available for sharing all the time.
The number #1 thing that is downloaded from my PC are my nude Drew Barrymore pictures, and the Paris Hilton video. So, folks just aren't flocking to my machine to download D&D .pdfs.
I'm going to blow the whole jerk/cheapskate, etc., theory, and add myself to a brand new category - the collector. I just collect these things. Sort of like, my downloaded collection of SchoolHouse Rock videos, and Ambiguously Gay Duo videos (which are free, anyway) that I have.
My computer is a toy to me. I work on a computer all day at work (well, except when reading ENWorld), so when I get home, sometimes I crank up my 80s music jukebox, and play MAME games and Atari games on Stella (combined, I have over 1000 roms).
To be honest, I've downloaded things that I've never even read. It's highly possible that I have downloaded things that I've never even *opened*. I just have them. There's really not enough time in the day to read through all these things. But, if I'm working on something, and someone says, "check out such and such book", I can, because it's sitting in my /downloads/documents directory.
And yes, at least one of my downloads resulted in a purchase - Complete Warrior. I was impressed enough with Complete Warrior that I purchased it after reading through the .pdf I have of it.
Another argument for downloading D&D .pdfs is that some things are out of print. My friend asked me to download Death in Freeport (I think that's it) - the 1st module in that series, because it was out of print, and he wanted to look at it. We found it on Ebay, but the guy wanted $100 for it. (seriously). So, I downloaded it, and gave it to him (he owns the others in the series). I don't know if he'll ever run it or not. Time will tell.
And for the record, I've never seen Sigil's stuff out on Bearshare. I also have purchased .pdfs from RPGNow. I own all of Sigil's stuff (I think), and a few other .pdfs that have interested me (including some older WoTC titles, and Book of Templates).
I would also contend that, at least from the perspective of Bearshare, some of them are hard to find. Some are fairly common (like some of the major WoTC titles), but some of the obscure d20 stuff is really hard to find. Lots of things aren't out there at all. Someone has to sit down and scan it all in, after all. So, whoever is doing that really needs to respond to this thread.