It's fine if you dislike it, and you don't even need a reason for doing so. But at high levels BLM has reliably forced procs and at least 30% full mobility from instant casts and also access to short cooldown teleports. This makes them more on-demand mobile than RDM, whose long duration mobility is only useful if the movement allows 5s in melee range and requires a ramp-up of 8 dualcasts (40s). BLM also has a lossless shield, which makes them more survivable than RDM in high level practical applications.