By your logic PLD shouldn't have Clemency in its current state, and skills that are not "useful" 100% of the time shouldn't exist. It's actually okay to have skills and spells that aren't useful all the time as long as a) they contribute to the thematics of the job in question and b) can serve some sort of purpose in overall function of the job.
It's worked in the past, and it would work again if SE chose to go that direction with RDM (sadly, they aren't).A DPS with a healing spell is such a sadly redundant thing in 99.9% of scenarios that I wouldn't want to see RDM celebrated as a "hybrid" when that really means "half-assed."
Having access to a healing spell wouldn't take away from RDM's DPS potential. Especially if you attach the used of the heal to a mechanic that costs the RDM DPS.Anyway, I really doubt that the devs would do this; they seem to understand that a DPS has to be able to deal damage first-and-foremost, so they'll probably keep the bloat of quasi-useless flavor abilities down to a minimum.
Since I seem to need to bring this back every now and then, we have the Art of War mechanic. A proc that makes the next nuke or heal instant cast, and only procs from melee hits. The result is that as you hit things with your weapon, you get procs and decide how to use them. Use it to insta-nuke the target and keep up your DPS, or use the proc to toss a quick heal before returning to hitting the mob. Bonus being that the class that had this mechanic did not have severely neutered DPS compared to the other melee DPS, despite having heals AND having barriers that could be placed on party/raid members.
As I've said, it works; I wouldn't be an advocate of it if it didn't.