That's a good point, but I don't know any other way to curve this issue.

Maybe a few dungeons that are DPS specific where there isn't really a need for tanks or heals, but rather knocking obstacles out of the way fast enough determines your success rate? At any rate, this sounds fun in itself. Perhaps, healing specific dungeons that are like the WHM 50 quest that you must keep a target alive with 4 heals.

Perhaps the real need is variance in the dungeons. 8-man 1 Tank/1 Healer/6 DPS? Changing up the party quota would be interesting.