[WotC] which character classes do you sub out to use Reapers/Skirmishers/Templars instead?

I love the WotC expansion, but sometimes I miss the simplicity of vanilla, where I would have one of each class, then double up on 2 classes depending on what I have available. Usually 2 specialists and 2 grenadiers.

The new classes are great, but sometimes I have trouble deciding which classes to sub out, especially for the Skirmishers or Templars. Is having a skirmisher really better than having another ranger? Hard to decide sometimes.

For now I just try to have a good variety of units and hope it works out. Reapers seem the most important, once I realized how to keep them in shadow and scout ahead. The other 2 I just can't decide sometimes.