No, I wouldn't buy a copy for someone else (unless it was the significant other), mainly because I don't have money to throw out of the window.
They can always borrow a book, and the SRD is free on the web.
About the reason for said lack of books:
New Players: You can always lend them the book, and of course you can sit down and explain. If they are interested in the game, they might pick up their own copy
Casual Players: Waste of money, they won't read it, anyway.
Too poor (students, unemployed): I'd consider that the only reason to buy a book for someone that isn't a close relative or loved one. If you have the money, It's OK I think. I would limit this to the PHB, though. That's all you really need, the rest they can borrow.
Just too cheap to buy books: No way in all the hells. If they're too cheap to buy the book, their interest in the hobby is obviously not that great. The excuse "but this costs me 30 bucks" doesn't really count. After all, roleplaying will grant you countless hours of fun, a single night out might come more expansive!