I main SCH and my main caster dps choice was BLM for a very stupid reason: since I play on gamepad both my SCH/SMN hotbars are very similar and since I play SCH for most of the time whenever I switch to SMN I end up pressing wrong buttons thinking I'm still on SCH LOL.

On a more serious note though, like some people already said SMN requires a lot of pet managing and button pressing which I already do a lot as SCH, so I like playing BLM to change the pace a bit. I also feel I have more control over how much damage I'm doing as BLM than as SMN because I don't have to worry about pet's dps.

And the feeling of landing a crit Fire IV that kills the boss is awesome.