If movement is insane, I just stay away from Fire IV until movement required slacks up.

It's just a matter of deciding when it's worth it to stay away from Fire IV, just like how BRD/MCH has to know when it's worth it to stay away from Gauss/WM.

The DPS hit taken actually ends up being smaller than trying to Fire IV with every other cast being interrupted.

BLM has always been a turret of a job, so I don't really think Swiftcast needs changing. BLM, like BRD and MCH, just has to know when (more) mobility is worth having over the stationary rotation that's used in more optimal conditions.