Recently there have been various blogposts about this, especially where professions are involved.
Markco likes levelling alts, and recommended using them to gain a variety of tradekills.
Gevlon thinks that levelling an alt (for profession's sake) is a waste of time and you should concentrate on a main toon, especially as the opportunity cost of levelling an alt is so high (100 hours played at even 200g/hour doing dailies or farming is a lot of gold).
My take is that the best reason for levelling an alt is so you can tank or heal (all toons can already DPS). Even if your main is a ranged or melee DPS, understanding how tanks and healers play will make you a better raider. For most players becoming the tank or healer is the best way to understand the role. I have my tank, healer and DPS toons, but I am levelling my 5th 80 because I can (not because I need to).
Your primary raider should have two 'primary' professions at maximum level (i.e. in addition to cooking, fishing, first aid or archaeology). You should know what your bonuses are, and use them. In most instances a raiding toon should have two tradeskills (instead of gathering), but consult a class/role forum or website for specific information.
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