I leveled Engineering for the same reason Bane did. I thought it was fun. Truth be told, I one of the few reason I did it(much like rolling undead), was
underwater breathing. In addition to that I thought the parachute cloak was the shit. Not to mention repairbots, mechanical squirrels, spyglasses, etc. I really enjoy engineering and I'm sad to see it nerfed so hard, but I prefer them ban it in arena than break it completely. I leveled that shit legit, no LOL MAKING LOTS OF BULLETS shortcuts. Just casually leveled it as I went (till maybe WotLK, then I may have powered some of it to get goggles or mailboxes/jeeves).
All in all, <3 Engineering.