I leveled alts in BC because I enjoyed playing WoW and my "main" character was leveling with my friends.
I leveled alts in WotLK because I enjoyed the different classes.
I leveled alts in Cata because I was bored.
In Mists, there is really to much to do for me to worry about alts, especially since I have stopped playing as often as I used to. So yes, technically the alts I have right now are 80-85 crafting alts. Though I seriously doubt I would eliminate any of them if I could have all of my professions on one character...
1. Gathering is a waste of time, so this gathering alt is simply an auctioneer for misc. crap I pick up while questing. Having gathering professions on my main character would just give me something to do while waiting for raids or queues...times when I am otherwise incapable of completing quests.
2. Bag space is not cheap. On alts, I have the option of guild banks that I can share on the same realm, but my main is part of a guild and lacks access to the guild bank except for raiding materials to better the whole guild.
3. Right now, I have gems and alchemy transmutes (for more gems) on one character, another is my herb storage and elixir master, my potion master is parked in front of the BMAH since I rarely need potions, my main is an enchanter for group DE, and my scribe is my second 90, a monk I leveled but mainly only use for the occasional wild pet battle. Access to all of my professions on one character would be nice, but I don't really have professions for self-sufficiency, I have them to make gold, and separate characters would still be better for organizing materials and tracking what I have and haven't crafted.
4. As Zachfu mentioned, the "serious" crafters with alts likely have multiple characters for cooldowns.
5. As Icpitl mentioned, if everyone was self-sufficient, there would be no market place. Making gold would become merely a questing grind for buying things off of the BMAH or random world drops. The most successful gold makers would be the bots with multiple alts completing dailies 24/7.
6. I believe the ideal situation is for every character to have one profession, cutting down on a lot of competition in the AH. The individuals who want access to all of the professions on one character are generally to lazy and cheap to interact with other players. They believe farming their own materials makes them free and that crafting is simply the pushing of a button.