First of all, I, like you, am a bigger rider. I'm 6'0" and just this morning I'm down to 215 lbs. Yes, you will go through bbs more often, but 1200 miles is too soon. I suggest buying the tools and learning how to do this yourself will save you tons of money in the long run! BB tool maybe $15.00, crank puller maybe another $15.00, new BB another $30.00 (for a good one, less for a lower model) Initial cost $60.00, compared to the $50 - $55 the LBS will charge to do it for a cheap one! You can break even on the first one!

Secondly, the bb that was on your bike was probably below the bottom of the line. Manufacturers save money on places most people won't recognize. The bb is the first place to hide inferior parts. An upgraded bb should last significantly longer. For you I suggest staying with a LX or XT level. The XT is close to bomb proof!

Thirdly, (and I'm suprised D Alex didn't touch on this) that the longer lenght will increase the lateral forces on the bearings. The longer the lever arm (spindle length) the greater the lateral forces on the bearings. Once again, an upgrade should minimize these forces.

If you're looking for reasons to get a new bike, I think 1200 miles in 6 months on a hybrid is reason enough!
L8R G8R