Crafting on alts
Nope, crafting on alts is fine. I have my main master two professions, one alt two more, another two more, etc. It gets the XP to characters who need it, and instead of talking to an NPC and paying a fee to change my active crafting disciplines, I just have to change characters.
There’s nothing seriously wrong with teaching your main all the crafts, but I think it’s generally nicer to spread it around on alts.
I have most crafts max on my main. I also have double dipped on a few alts (actually, everything is spread between all characters EXCEPT Jewelcrafting Tailoring and Armorsmithing, all of which are on my main only). However, my main also has 500 Artificing, whereas everything else is at 400, so… problems arise. If I could do it all over again, I’d have spread it out a lot faster… ._.
One of the nice things about crafting on alts is that crafting experience boosters don’t run down when that character isn’t active.
So you can pop a booster and craft as much as you can with the materials you have available, then log that character off and come back later without losing any of the booster time.
It also takes money to switch between crafts (not a lot, but it could add up) so it makes sense to have two crafts on four different toons.
Craft on an alt because it levels the alt up — something your main doesn’t need. That way the “gameplay” that is crafting can be put to some use.