Personally I think it's worth it to do all the gathering professions. It's easy enough to level them with what you just happen to walk by, and that's quite a bit of gold you're just leaving on the ground if you don't. I went with herb/mining on the main, and skinning on a clone since you can /assist the main and then /cast skinning to skin without ever switching over.

Crafting professions on the other hand are generally not worth it, except for enchanting, which is a must on one of your toons. I chose to level Jewelcrafting as well simply because it's the only profession I've never done.