I think it's just because healer's doing dps is a double edged sword in terms of views of opinion. In circumstances of dungeons involving capped ilvls, a tank losing hp rather slowly, or dps not meeting the expected damage for the mobs would encourage some aid from the healer as far as dps is concerned. But it's for the very same circumstance of dps's expecting to meet the quota that encourage just as much distaste for some people, when a healer focuses more on casting a damage skill rather than a heal skill in some circumstances.
Not all dungeon runs are the same, results always vary. But people will still have their opinions, usually more in particular to their own experiences as opposed to another who has potentially shared opposite of their results.