I'm a caster. And while as a RDM I do envy range for their mobility for instance on E3S (see the logs it hits RDM hard that fight), what Telos said is right : if even with that mobility taken into account, a BLM or a melee is still 1k5 above you in rDPS (so also accounting for raid buffs), why take you ?
At similar skill level, if a BLM can die twice in a fight (not consecutive though) and STILL output a better number than a RDM or a DNC not dying, something is fundamentally wrong balance wise. Cause rDPS takes everything into account. I t means either BLM is too strong (but nerfs are rarely welcomed so not gonna happen), or the mobility tax / the raise tax is way too strong on others.
I play my BLM worse than my RDM but starting E3S I'm considering using it to help with enrage cause I feel like a burden for my team as RDM DPS wise. Already started a bit on E2S.
If we need that 1k rDPS more I'll probably need to.