If your primary profession is either Blacksmithing or Engineering, Mining is practically a prerequisite. Both professions require materials that can only be acquired through Mining. You could choose to not take up Mining as your second profession, but consequently you’ll have to spend an inpractical amount of money on the materials you need to raise your Blacksmithing and Engineering.

Mining is a great way to make money compared to some other professions. Unless you intend to take up Leatherworking, Mining is a good profession to take consider. The only way to make large amounts of money through mining is by selling Arcane Crystals and Blood of the Mountain in the Auction House. Both of those items are extremely rare however, and you shouldn’t become a miner solely to farm them.