In addition to the previous - there is a problem with current terrain GF specialization: it's nearly useless mechanics, because there are too much terrain types, so very small chance that some good part of current terrain-specialized formations will find another planet with the same terrain to be conquered before this formation will become outdated, and so terrain-specialized formations are really nearly one-time things. Combined with their cost and the fact that cost modifiers are cumulative - it is inevitable that players have no desire to use these specs.
I see several good ways to resurrect terrain specs:
1. Reduce their cost. (I'd consider to add, say, forest-spec to some of my first line formations, if it's cost will be no more than 10%.)
2. Make it's cost reducable (down to zero) by the terrain of home planet and the terrain of decent garrisoning / combat experience.
3. Add unit update mecanics, so we'll have a way to remove specs, that are useless now, and add what we need with the next big battle.