in a lot of ways, the shield gives the other healer a bit of time to lay on a heal, because people will -always- stand in things.

in 4 person dungeons, one may have a big heal, and a heal with a regen, while the other has a big heal and a heal with a shield. unless they change that, I do not forsee a lot of "obsolete" issues. the shields, even applied as part of a direct heal, are bigger if a crit is involved, but they dont last forever and really, at the upper levels, may be good for a single hit (if that)

if I recall, the disc shields were rather substantial