DisruptorX is right. Dragoons will probably be used to quickly take and hold key areas. We don't see this in the demo because no key areas need to be held and we get very few of them.

I can imagine using them to set up suprise ambushes or to circle round an enemy and cut off his escape routes. As well as taking strategic buildings or hills and other battlefield landmarks. They can also be used like cavalry archers, which is handy.

I think the reviewers just weren't imaginative enough when they said there is no reason to use them. A units usefulness isn't just about how high its stats are.