Imagine you queue into Stone Vigil with a level 90 whm. His Cure 2 heals for, say, 1k. You can do big ol' pulls, you got Tetragrammation and Assize and Benediction and Benison and Holy and like 5 other helpful buttons. You can do big, reckless pulls with no worries at all.
Now imagine you queue into it with a level 40 whm. He has Cure 2 for 1k and nothing else.
The tank does a big pull in the first instance, it goes smoothly, everyone has a good time. The tank does a big pull in the second instance and dies. People blame the healer.
And before you think, "oh people won't do that" then you clearly haven't queued for MSQ recently. A lot, and I mean a *lot* of people drop Castrum at the drop of a hat because it's less efficient. People WILL kick the newbie healer if getting a higher level one could make the run smoother.
Now, extrapolate that to every role, in every dungeon. Got a level 22 archer instead of a 90 bard? Boot him, get someone who does good DPS. Level 31 paladin instead of a 90 paladin? Give 'im the boot.
Also think how confused a new player will be. Let's say you get a 90 warrior in Haukke Manor and he can pull the entire first floor and take no damage. As a newbie healer, you might think, "wow, warriors are super strong!" The next time, you get a level 35 warrior, he does like 2 pulls and dies because you were expecting him to be invincible so you never healed him.
The very minor inconvenience for higher level players isn't worth ruining the experience of new players.