I thought it was common sense that engineering encouraged teamplay. It encouraged the team to split into 2 groups, one working on the catapult or siege tower and then other buying time for the completion of the siege equipment. It also encourages teamwork in the enemy team. It encourages them to zerg your team in 2 battles of 30 vs 15 (on average, but similar ratios). Which almost always results in the team that split in two losing.
Yes, you can try and tell people to stay behind and defend, but not all will listen. Also, camping spawn is very frowned upon. You all bitch and moan when an archer camps a roof because it delays the round, or when a team camps the hill 2 feet from their spawn, or when a HX runs circles until the round draws, what do you think building a siege tower or catapult does? Did you learn nothing from how many times ATS would draw rounds by building siege towers?