I like the fact you can build them instead of just selecting one.. It also use multiple players/time which adds another aspect to the game. So the ability to build one shouldn't be removed.
If they did add the ability to pre-make some siege equipment I think there would have to be some restrictions/conditions.
- Only for sieges
- Only attacking side
- Only Catapults and siege towers can be pre-made
- They spawn right at the back of the map behind spawn (players have to move them during the siege if they want them moved)
- They should come at a much higher cost (to compensate for the saved time/player requirement)
- Cap the amount you can have pre-made (2 catapults and 2 siege towers, no matter size)
However, I think this isn't really necessary and there are other more important things the devs should be working on first.