Judging by all of the information I've received:

I know how speed running this works. SMN/Whatever gets it done about 7-9min. (personal records) If every minute counts, why waste that time by looking for specific jobs? Especially during the low times for server activity? You can take me as SMN or you can spend another 10min looking for a BLM.

The answer to my question, BLM has unlimited MP and higher AOE damage. It makes sense but the reasoning for lock outs just don't really jump out at me. Thanks for the feedback. ^^