Lol you think something has to be usefull to be bought? Just look at the enviromental books. Very few campaigns are set solely in one enviroment and even the ones that are did just fine before those books. I ran a campaign based on a northern tundra enviroment with complete accuracy 6 years before frostburn came out. How? I got online and looked at national geographic for arctic enviroments, read everything they had and added onto the rules where absolutely neccesary. Oddly not much had to change. Just my random monster tables and some of the rules for exposer had to be made more severe. Did i need to spend 40$ on another book? Hell no. I flipped throught that book. Not a single usefull idea. I am running a semi arctic campiagn now and i still didnt see one thing worth using.
Did people spend money on frostburn? Hella. Thats why they made more. The whole book was a few items that will never be used, a few PrC's that suck, a couple of rehashed monsters from 2e and a few spells.
A lot of people buy new books just to save themselves the effort. I think that DM's buy most of the books, players buy a few if they see something they want to use in one. And most of them are bought by people who thought it looked neat and didnt really page through it.
Players handbook 2. Will they make it? Yes. Will people buy it? Yes. Will it be even remotely neccesary? No.
They will rehash old spells from older editions and other books, they will reprint classes, equipment, and probably PrC's from other books. They will give us 50 more feats that add +2 to this skill and +2 to that skill, when we could easily let a player take a feat and add +2 to any 2 skills of thier choice, making all those feats useless. They will reprint some of the spells from books we have allready bought, but not enough of those spells to make us not have to flip through 10 books looking for a specific spell because we want something new.
Will you need the PHB2? No, not at all. It wont even make your life any easier. It will no doudt have some broken class, and probably a broken PrC. Just to make your life harder. Will you buy it? Maybe. Will some tool in your group buy it so you have to deal with the consequences? Hell yes. Live it, learn it, love it. WOTC hates DM's. They have done thier best to make use non essential, and when that failed they have tried all the tricks up thier sleaves to make our lives harder. And they will do it again with a players handbook 2.