The Burning Crusade Hunter Primary Skills
2.1 Primary Skills
2.1.1 Engineering
For early brackets, engineering is excellent for PvP because it gives us access to unique items that no other class can use. For other professions, we can buy all/most of their items on the market.
For example, in the 10-19 bracket, with engineering you can craft a good quality head piece ( http://thottbot.com/?i=1369 ) for both armor and stats whereas most other players would only be able to get a headpiece through a lot of pain. Later on, you get access to a wide assortment of very useful trinkets and explosives that -- again -- no other profession can use. Dynamite and bombs for example are very useful, since bombs can stun many players allowing a flag carrier to escape, and dynamite will add significantly to your DPS.
Furthermore, engineering lends the ability to make engineered bullets, which are superior to vendor-bought ammo at the same level bracket. You could buy engineered bullets at the AH since they don't require engineering, but that tends to get rather expensive.
2.1.2 Mining
If you take my advice to go engineering, you may want to choose mining as a natural pairing with it. Really, any gathering professions is a good tools for making money, particularly skinning and herbalism. That said, I don't even believe this is necessary, because material prices tend to fluctuate greatly so if you buy low and sell high, you can make a profit out of those without having to gather them yourself. And there are always people who materials on the AH at a total bargain because they don't realize the actual worth. As a note of comparison, my hunter had over 1000g at level29 and close to 3000g at lvl49 just from this type of strategy. You do not need to be a skinner to buyout underpriced light leather and resell it at a high profit.
On that note, you really do not need to have a complementary gathering profession to your crafting profession. It's all opportunity cost -- if you mined copper and you use it to make an item, then you're paying the cost of having that copper because you could have sold it. This is known as opportunity cost. So you're no better off than if you had just bought it at a cheap price on the AH. What's more important is that you have at least one gathering profession so that when you're out in the world, you can be gathering materials for free whether they be metals, skins, or herbs.
2.1.3 Others
At level 70, each profession affords different advantages that need to be carefully weighed since most professions give us access to BoP items that are unique to that profession. Blacksmithing gives us a 2H weapon, engineering gives us goggles, leatherworking gives us armor. Which profession you settle on may just depend on what exactly you have access to normally given your auction house and guild progression. For example, you will probably not need the epic gun from engineering if your guild has progressed into later Kara content.