The point of the new forest type would be to hinder vehicles more than regular forest hexes do without hindering infantry units more. Adding any type of terrain to the existing forest gives additional movement penalties to both. In fact it almost makes it worse for infantry since it effectively reduces their speed to 1 hex.
This is a big deal that I thought should have been added a long time ago. I was surprised that they didn't put it into the very first Steel Panters, and I think it should be the first thing they put into any new upgrade.