I don't like the combo myself because then I can't really heal myself when I solo, but here are two reason why I don't care:
1. Skillz > Race/Main Job > Subjob
If you are good at the game, then you can make any combo of jobs work (except maybe DRG subbed, but that's my opinion only :P) I've seen high level BST/BLMs, so it presumably can work. Athough, if you suck at the game, you will suck as BST/BLM. In fact I'm always the first to defend a job combination. I remember a heated argument where I defended WAR/WHMs and people pretty much called me a moron.
2. I don't pay your bills
Hey, you pay the $12.95+ a month to play, so I can't really make you do anything at all, only express my opinion. I pay my money every month, and I'd rather enjoy my time, not be miserable or compromise my gameplay. You (or parents, or w/e) pay your bills, so if you wanna be a BST/BLM let your money and skillz do the talking and have fun while everyone else wastes time arguing or b*tching at you.