Personally I think it's a weak way to play the game. I mean, ignoring WoW's faults, there is a game that's supposed to be played and they're skipping it. I found that in WoW, it wasn't particularly damaging to the game, as respawns are quick, and you could still level/make money/go about your business.
In other games, particularly FFXI, gilsellers were a serious threat to the game - they'd camp spots in sought after areas (there are a limited amount of areas to level up at each point) and farm for hours and hours, stealing mobs. Some were okay, some would heal you on the odd occasion, others would deliberately lead enemies into your group so you wiped, while they took your spot. With the NMs, which would take hours to respawn sometimes, they'd be there immediately to pick it off, get the loot and dump it on the AH, which was a major pain seeing as making money in FFXI is near to impossible without cashing in on some of the high-selling items. Such as the Leaping Boots.
I can understand why people buy game currency - but I do feel that botting and gilfarming can be a really lame or damaging aspect of a game, but that depends on the game really.