some of the AVA terrains have destructible bridges. the problem with destructible bridges is that it halts the action. so to have a tank fight going on by a bridge and its the only way into a base. kill the bridge, kill the action. if you're looking to blow up bridges from the air, well thats different but still stops all ground action.
part of the problem with building terrains is if the game changes, so does the terrain. we had some awesome terrains in AH2. twin rivers was a fun one, there were tons of others, many with custom airfields and custom cities. when hitech changed the game it fried the custom stuff. who wants to put in hours of work, just to have it wiped out by a game change?
the AVA team had one terrain where you spawned with a tank onto Omaha beach out of a LST. it included a custom Eiffel tower where paris would have been. that got wiped out.
skins are great, and dont seem to change with game changes. so those are all stable.
just my 2cents.