Well having the buildings come tumbling down creates quite a few problems.
Firstly, respawn points. Im pretty sure its hard to respawn under the rubble of a destroyed building. Even if somehow the respawn points are taken away (which would require some unnecessary scripting), the buildings a player can spawn in is less. Imagine if you only had one building left, a couple good guys from the other team can do A LOT of killing if they know exactly where players will be coming from.
Secondly is the purchase terminals... these are scripted windows, and again would take a chunk of work to make these disapear.
Thirdly, destroyed buildings are still used as a strategic outpost for the defending team. Many times I've been on City Flying, where we'd only has a barracks left. I'd ask the players to get good anti-vehicle units and perch themselves in high, strategic locations, including destroyed buildings.
Fourthly, they are a strategic outpost for the apposing team. Again, on City Flying, stealth tanks can reach a destroyed Weapons Factory without being hit by the AGT. Since they use it as a "barrier", they can make their way to the refinery, or even the powerplant, without being hit much. If the buildings collapsed, you wouldn't be able to do reach certain places in the enemy base when defenses are up.
Not to mention, destroyed buildings are the perfect places to hide in/by if you're alone in an enemy base with a sniper, or a nuke.