Now, as I understand it, you can quite easily use any models in Maya 6 and import them back. This way, you can limit yourself to models from games in the Heroes series, rather than using a ton of mismatched models pulled from everywhere and downloaded from some random site.

This might not be entirely relevant, but since it's being discussed, I think it's worth adding my two cents.
You can take models from different games, but strictly for one faction. For example, if you're creating demons, you can extract them from one game so that the models maintain a consistent style, and in case of any issues, they have the same problems with animations, etc. (Uniform and unsightly, so to speak).
You could pull models from different games and create different factions. For example, I know you can extract models from Eador. Masters of the Broken World and from Disciples III Reincarnation.
But again, this is just if someone wants something new, but doesn't want to delve into old sources.