This would be horrid for transpose lines, cause at times its optimal to use transpose within an rotation,
In general I can see where you come from, but BLM is most mobile out of a majority of the casters for the reason being they can use it when they deem so.
RDM while every other spell is instant, still has to cast, and melee combo requires melee and you can store up to two, but you cant hold 2 cause you'll overcap
SMN while having mainly instants within its rotation, you do have to hardcast eventualy, like BLM, and there isnt any way to skirt around it.
For example in p10s, a mech called shadowed messengers summons two adds so i need to be on my side to avoid then solve the following mech. as a BLM my Leylines come up then so I pop them. then right as they end i use a xeno or thundercloud and Zooom over to my light party, then i use another instant and cast triplecast and solve the rest no problem. The thing is, I had to learn the fight and learn what, where, and how I can use my tools to my benefit. Thats what fun about BLM, imo.
others here have shared great insight as well!
