Blaxer put into words this issue beautifully, a job very well done

hchan1 also highlighted a factor that makes dual units both a blessing and a curse with their need of micromanaging.

To me the better hybrid units are those that have a passive change in attack such as Priest Warriors and Dark Priests and both also have a heal priority skill. All in all an attacker to healer is always a good thing, the other way around not so much. Actually the only class I know of that does that are Dragon Shamans, though I think most people field them for their attack oriented skill rather than their non-skill healing.