The main problem is you have 5 to 10 people messing around at building stuff while the rest of the attacking team try so hard to get the flag ... it makes an even fight if 1/3 of your team are building useless shit outside (healing tents are pointless as attackers, dying heal you faster, you respawn after 3 secs with full ammunition/HP). Most of the time the siege tower is bugged and useless. The only advantage I can see if to have an archer up there shooting at the wall, but then again , after mid round , it's useless since most of the fights happen by the flag or on the way to the flag, not at the wall anymore.
I still think unless it is really fixed and useful, that siege tower remains prohibited.
Catapults just bug maps and also you'll have plenty of people playing with them without really helping your teams.
Spawning with only construction sites to build stuff, in my honest opinion, isn't participating in the battle to help your team to win (as attackers) and should be considered almost like leeching.
But like we mentioned so many times , this is the BAN / UNBAN request section, if you want to discuss the matter of siege towers, this isn't the right place to do so. General Discussion or suggestion would be more appropriated.