And that right there is the rub for me and my group. When we switched from 3.0 to 3.5 there was a lot of grumbling and grousing, even though I think most of us accepted the reasons for the switch. But even to this day we
still find odd rules changes tucked away in subheading and side paragraphs from that switch. Argle!
The one guy in my group who was interested in Pathfinder (and not 4E) promptly changed his mind when he was reminded of having to switch from 3.0 to 3.5. Why go through that again? If I wasn't going to play 4E, I'd play 3.5 so ... why is Pathfinder needed? (For those saying to support the AP, why make changes? Want your main rulebook in print? OGL PHB anyone?)
Or just play 3.5, which is fine. But my group, and I think we're not unique, has definitely had the rough edges of 3.5e wear on us. Our campaigns were steadily escalating in weird factor in search of new experiences within the ruleset (to Eberron with characters who were orcs, warforged, bugbears etc., to Planescape with characters who were bladelings, genasi, etc...). The improved beef of the splatbooks meant that core classes, spells, and magic items were largely obsolete. "Screw that, I'm a swordsage!"
Pathfinder freshens it up a bit. It fixes some problems. It brings the core to a comparable power level. It keeps a PHB in stock in a way that will actually generate precious revenue for the company doing it.
The trouble with 3.5e is it's hard to develop it further. All the content in all those splatbooks isn't OGL, so you can't use it, and in fact if you get too close I'm sure the WotC lawyers would be happy to go after you. With a pure fork they have a lot less to worry about there, and more freedom.
And I have to agree with the other poster about the cred Paizo has with me. For a long time I've really admired their work; they carried the flag of Greyhawk and produced great content. It sure wasn't WotC-core adventures that kept me with the game (bleah!) or Dragon Fetish Splatbook I-III, it was the stuff in Dragon and Dungeon (and the stuff that people like Green Ronin, Atlas, Mongoose, etc. were doing). They "speak D&D" in a way that for me, the 4e guys don't. Everything they've done has been the highest quality and I've enjoyed their APs and 3.5e rules work immensely. So I have very little concern about committing to the "Paizo D&D train;" they have a proven track record with delivering quality content that the current folks working at WotC do not. (I know, a side note to the "why not 3.5" question).
I think the change is needed - I would still play 3.5e with Paizo at the helm, but I'd be less excited about it than I am with Pathfinder. (I'll also note the open process in which they've conducted the playtest and allowed everyone to read their beta and comment on it/playtest it in depth is evidence that the game can continue to develop in a way the *fans* want...)