In general, I am Captain Homebrew (r)... but with a lot of provisos.
I do buy some campaign material. This is not because I necessarily intend to run a game in that world, but because the setting ideas, extra rules, monsters, magic, etc., has something to recommend itself to me. Sometimes I have only a "deeper" supplement, not the core -- with Scarred Lands, for example, I have Hallowfaust, but nothing else for the setting. I am quite willing to steal good ideas from other sources, cobbling them into my gameworld as feels appropriate. Ars Magica is probably the only line where I have gone out of my way to pick up most of the setting material, but even there I have fallen off the track on several occassions.
Equally, I don't usually buy adventures. I have looked at several and found that after adjusting the monsters, treasures, geography, interrelations, and architecture to fit my homebrew world, I might as well have started from scratch. I do own a couple Paranoia adventures, but that is for the amusement level. I also own a couple of others, but usually these have been bought because I am buying up some lot listed on eBay. The ones I bought for Ars Magica, with the exception of Broken Covenant of Calebais, have been universally disappointing.
None of the D&D worlds have ever really grabbed me, not Greyhawk, not Forgotten Realms, not Planescape, not Eberron. I am fascinated by some of the notions in Ravenloft, but not its execution. Now all of this is from a GM-who-likes-doing-homebrews-POV; I wouldn't turn down running a character in one of these if a friend was running it and I would never tell someone that they are somehow not really a GM if they use one. Overall these settings are popular, successful, and a lot of people love 'em. Fine with me. For my money, however, I prefer to do the homebrew thang. Thus I am not overly worried about support for anything along the D20 line.
Still, there is also the Economic Necessity line. Game companies need to turn out supplements if they are going to stay in business. A core rulebook (or even set thereof) will not keep a company afloat, no matter how snazzy the game might be. Since this is true, it is in every company's best interest to develop a setting with a good amount of support material and hope that people will become loyal to the line. That is good business sense, amongst other notions.
