Imagine a big house where defenders camp in, and attackers want to capture this house.
It’s obvious that you can’t capture that house alone, so you stick with your team.
If there is no house: defenders run around, counterpush, spawnkill etc. and that leads to the deathmatch feeling and running around alone.
I don’t see houses in the current maps, but houses are needed to encourage teamplay on both sides.
I made a longer post at the developer forums, feel free to comment
forums.warchest.com/showthread.php/43837-Respawn-system-and-map-design