I've done this - simply because I get bored.

Casting cure once every couple of minutes gets dull. Tanks in low level dungeons tend not to respect the ability of their team, and change their tactics accordingly - they just do it the same way every time. If a healer is being wasted, the tank is not doing his job correctly. We're there to heal, not stand about and occasionally dps.

If the run is noticeably faster and everyone stays alive, then I'm not too concerned about ruining the tank's pride. I'm going to go off and do it.

Just a note to remember is that if your pulls were too large for the healer to keep up, you'd slow down. Seems sensible then, that if your healer is seeming awfully idle, then you can increase the difficulty and speed. Until tanks discover this ability, I'll do it myself.