For me, its another one of those "it depends" questions.
HERO, my favorite system, has a core book that is really all you need...and then has sourcebooks that set up campaign worlds and toolbooks that let you round things out. And all you really need is the first book.
OTOH, as I look at my bookshelves of RPGs, I have all of the 3.XEd FR material...and a goodly portion of all the other books for various campaigns.
Heck, I've got 6'+ of RIFTS material (that's about creativity, not mechanical quality).
Sometimes, all you need is a nicely detailed campaign setting. Sometimes, the added books really do add something to the game.
And I think the deciding factor is quality. A lot of mediocre supplements does me little good, and I'm less likely to buy subsequent stuff that relates to it. A few great products has a coat-tail effect- I'll buy the top-notch stuff, and buy the other stuff on the strength of those superlative ones...to a point.
HERO, my favorite system, has a core book that is really all you need...and then has sourcebooks that set up campaign worlds and toolbooks that let you round things out. And all you really need is the first book.
OTOH, as I look at my bookshelves of RPGs, I have all of the 3.XEd FR material...and a goodly portion of all the other books for various campaigns.
Heck, I've got 6'+ of RIFTS material (that's about creativity, not mechanical quality).
Sometimes, all you need is a nicely detailed campaign setting. Sometimes, the added books really do add something to the game.
And I think the deciding factor is quality. A lot of mediocre supplements does me little good, and I'm less likely to buy subsequent stuff that relates to it. A few great products has a coat-tail effect- I'll buy the top-notch stuff, and buy the other stuff on the strength of those superlative ones...to a point.