Its not really a waste when your fighting mobs like slimes who has something like a 80% dmg cut from all physical dmg and the only magic caster you have is the rdm.
IF you know there redundant to melee damage, it would be stupid to go with 1 mage and that's your fault to begin with. Another thing to think about, Jellys have annoying TP moves, your RDM spent his MP nuking... who's going to cure you? Or whose going to pull hate off the RDM from nuking when your damage isn't gaining you hate?
Or when your simply fighting the clock to squeeze as many treasure-box out of a zone as possible. Convert merits allows you to do that little extra, withouth scimping on haste, refresh, enfeebs and what not. Rori handles it brilliantly and we can typically walk out of a zone with some 15 coins each and atleast one AF each (given that the right types drops), but usually more.
So your saying, he can't refresh himself, haste you, cast a few enfeebles and still have a lil bit of time to rest? I can see nuking on Jellys cause of the damage reduction, but otherwise its waste of his MP and both your time nuking on other mobs. Good that you duo Limbus if thats what you want, but I have main healed a 6 man Limbus group (With a BRD to aid with ballads <3 Dejan) and refreshed our BLM and still had time to rest and kept up enough MP to last, even when HP/MP chests were being stingy to drop.
I really don't see how an extra 1 min 20 seconds is going to mean that much ina party situation, MAYBE in a solo/duo/trio, if your going for constant chains or a time limit, but overrall it is kinda pointless. I'd rather have just a little bit of umph to make my important spells land. Cause when soloing, I'd rather be able to bind and gravity and sleep my enemy much better then to get MP fully back (and maybe get whacked while converting 'cause my bind or gravity wore much faster).
But in the end, merit your RDM how you want to play it. Don't merit the way every RDM does. Play it to the way you handle RDM, and what works best for the situations you most likely find yourself in.