I was thinking about this issue after I woke up (thinking about WoW seems to go on even when Im not playing): What are probably the most effective class/profession combos?
I know lots of casters like Engineering because it gives them early access to very good head items (goggles and the like). Ive also heard Paladins can enjoy Engineering so it will give them a access to ranged attacks, something they lack.
Someone in my guild plays a Warrior, and he jokes about being the only non-caster in the alchemy shops. But Alchemy gives a Warrior some pretty good abilities they lack solo- in fight healing, and buffs.
From my own experience as a Tailor, I can see a person would, from a monetary standpoint, be better off taking gathering skills than a craft skill like blacksmith, leathercraft, or tailoring: you will make more (and more steady) money selling the results than you will by hawking your wares in trade chat, and without the added 'bonus' of the skill also being a money sink. But I digress