The problem with this is that even if you do manage to make that catapult in under 3 minutes, it does NOTHING to capture the flag. Sure you kill 3 archers on the wall before the round is over, but that means 3 people got 1 kill each in 6 minutes and were no where NEAR to the flag to cap it. This is all supposition that you will actually be left alone by the other team to make the siege equipment and that the castle your attacking will have camp spots that are targetable to your catapult.
If you want to practice your siege building for strat purposes go to duel off to the side and do it. If you want to contribute to your team and win a round, gear up and get into the fight.
TL:DR - Building a catapult has NO effect on the outcome other than your team being down 3 warm bodies.
We generally only use 2 people acting as crew, but what you are saying still seems a little prejudiced to me. I get what you are saying, that the crew is not helpful to the team. But let mem direct you to one of the rules:
OK: Being a bad player and getting no kills
Of course, you already knew this, you are an admin for christ sake, but if there are bad players allowed to go around doing bugger all for their team, then why can't there be a siege crew that, still gets kills, but apparently to you , still does bugger all. Even though there are player who will go around at 0-8 not even knowing how to play the gamemode, they do not get kicked, and even the players who are not new, but still play terribly, they do not get kicked.
If you wish, write us off as terrible players, but truth is, as it says in the rules, there is nothing wrong with being a bad player.
And so we should be given a chance, or even more so, the freedom to play the game how we enjoy it. Who knows, it might catch on and people may actually start to become pretty damn good at using catapults.