You can do whatever you want if you're solo'ing. Deft was referring to you rolling up as /RDM when someone else's time is in question.
While I think Deft could have said it with a bit less grump, the point stands. The question was specifically what's a good sub for soloing. /rdm wouldn't help your drain enough to matter, and the minor cures you'd be capable of wouldn't be great at that point. Pre level 24, much better to sub something that helps your offense and/or defense, such as /war, /mnk, or /drg (post level 20). This will speed up kills, then you can rest up your hp as needed (gelong staff and regen drinks will help). Then at 24 you can /nin and throw shihei at the problem. Don't need cures if you don't get hit.
By all means though, you wanna do a mage sub, do it up, have fun with it. But I've always preferred /whm or /blu for that purpose. /whm gives you more mp and slightly stronger cures and sneak/invis, /blu has some fun spells and decent enough cures.