I think the OP has it the wrong way round, as a BLM marking targets increases my DPS, no doubt about it. It doesn't make a massive difference but it's still information I can use when judging which targets to re-aply DoT's too and sometimes which one I should use for my single target stance flip spells or thundercloud/firestarter procs. Then consider as a BLM I have a cast time so if a mob is at 5% health there is no point throwing a spell at it if the other DPS is a good single target job, it will be dead before my spell finishes and I'll stand there like a nugget interrupting myself. If I know what the next target for the tank is (I can't read minds and different tanks do things differently) then I could be doing something constructive to the next focus target instead.

A party that's all on the same page and attacks mobs in an intelligent and timely fashion will always run better than one that allows chaos to decend on it, marking makes the former far more likely so why not do it?