For me, it is solely a question of whether or not the players need the rulebook for us to enjoy playing. My criteria for that condition, however, is probably not the same as most DMs.
The players in our group have no problem playing pregens characters, for example. So if I want to whip up a game of Dragon Quest, then it won't matter that I've got one, out of print, copy of the rules. I'll hand out pregens, and we'll learn as we go.
It's really kind of odd with us, because the simpler the rules are, the more likely the players will want to read them/monkey with characters themselves. So they are more likely to want their own copy, unless it is something as simple as Toon.
Naturally, how much "looking up stuff at the table" is required, affects this too. I'd be more likely, for example, to run out of print, with one copy, of D&D 4E than 3E, simply because with 4E I'm going to bother to get "powers" onto cards anyway, even without the online CB, but I'd hate to do that with 3E spells into the middle levels. And I could run Burning Wheel just fine with one copy of the rules, if we had about three copies of the character burner.