Which is a good question. There's a lot of good reasons to buy some things from vendors. I always keep an eye on what they're selling in terms of recipes and magic items-- Auctioneer will sometimes tell me that I can resell a vendor item for more on the AH, and that's easy money. And I have-- only once-- bought one of those white items from a vendor. My warrior was in Stranglethorn Vale, and I was leveling up so fast I'd forgotten to get a new weapon, so it turned out the vendor's axe was better than the one I had equipped, so I used it until I could make my way into an instance and get an upgrade. And I know some level 60s that will buy some cheap boots from a vendor, just to put Mithril Spurs on for riding.
But Undying's right, especially at the higher levels, the stuff the vendors sell seems, well, filler. Is there a reason to buy from vendors at all? Or are they just the garbage receptacles of Azeroth, destined to take your trash and never sell any of their own? And what exactly do they do with all those Bear Gall Bladders, anyway?