I think they could enhance the campaing by leaps and bounds, but thats just my opinion.
In terms of skirmish Edit: by this i meant melee/online ladder games as well, HELL NO. However, custom games are a must for heroes, it would give us so many options.
If heroes will be in custom games....I wonder, will there be a "generic hero unit" in the object editor to base new heroes off of ? It would save alot of time.