Essentially, while Blizzard states that they intend Blacksmiths, Leatherworkers, etc. to create items to sell for profit it usually doesn't work out that way. Because impatient people are willing to spend A LOT of gold on the Materials to level their professions up, the unregulated economics of the WoW Auction House make the value of those materials far greater then the crafted Bind-on-Equip item you'd make.

People take up crafting professions because they want the top-level items that they can produce. For example: A Warlock may take up Tailoring in order to craft the Frozen Shadoweave 3 piece Epic Cloth Armor Set. As long as the tailor is at 375 and is specialized in Shadoweave tailoring, they can make this Epic Bind-on-Pickup set that's better than any of the stuff even in Karazhan! As soon as they ding 70, they can equip this great gear, and have a great advantage.

It is important to decide what spec you will want at endgame before you decide on a complimentary profession. Let's look at Shamans: If a Shaman wanted to raid as an Elemental spec (yes, rare, I know) then Leatherworking will yield a great 3 piece Epic set. But if you're going to spec Enhancement and raid, Leatherworking is acceptable, but Weaponsmith(Blacksmith) may be more desirable to craft a pair sweet Maces for better dps. However, if you're going to go Resto as a Shaman, (VERY popular choice, due to the great Chain-Healing spell, and the demand for healers) then Leatherworking is a HORRIBLE choice! As a Resto Shaman, you'd probably want to go Herb/Alchemy to make those AMAZING Trinkets which really benefit raiding Resto Shammies.

Frequently, Prot warriors and Pallies drop Blacksmithing and take up Engineering. Blacksmithing is the Ugly Bastard child of WoW professions, and on a lot of days, Leatherworking is the only kid who'll play with it. LOL (No offense intended to the many of you ugly bastards out there, btw) It's generally agreed upon that Blizzard did Blacksmithing a GREAT disservice by not having good BoP Plate tanking patterns. I'm hoping that this will change in WotLK.

I'll suggest to new players to research which professions will be most beneficial to your class & spec. Google will be your friend here.

Hope that this helps.