Yeah, I don't see the need for a DM to buy up all the player crunch. Let the players do that. Borrow it if you feel like you have to read it to run those characters. 4th edition stuff tends to be pretty well balanced overall, so going over everything with a fine toothed comb isn't necessary.
4th ed is REALLY good about not forcing you to buy tonnes of books, despite the "everything is core" motto. The content is seperated pretty well. Budget minded players should just pick a power source they like, and pick up the new phb, av, and xxx power book every year. Those three purchases will cover about 95% of the options available to that power source every year. To get the rest, just do a 1 month sub to DDI every six months or so.
If a player likes to dabble in everything, they've always had to spend alot of money to keep up, and 4e is no different. Fortunately for them though, new classes are only introduced in PHBs and Campaign Settings. If I was one of these players, I'd probably just buy the annual PHB, Campaign Setting, and AV, and keep a year long DDI sub to snatch up all the options from the xxx power books.
Myself, I just plan to keep buying PHB, AV, and MP every year, with the occasional other book purchase(gotta buy Dark Sun this year). I can generally borrow other books if I want to read them badly enough.