I think comparing the spells list of a RDM subbed to that of a BLM or WHM subbed proves it all.

With RDM sub you don't get cure till level 6. You don't get your first black magic attack spell till 8. Protect at 14. Cure 2 at level 28 (by the time your that high the REAL RDM's will be cure III'ing everyone not to mention it'll drain away your 3 cure spells by that point if your going melee/RDM) And not to mention the almighty RDM spell refresh you can't even use till 82 (heck that level isn't even in the game yet.)

I think the spell list alone proves they aren't too well off. If you really want magic, sub something that'll give you more MP, better/more spells at lower levels, and some passive/active skills that will actually help with magic.