There are several very large flaws in this idea.
First, you are only considering evil vs good. In the competitive community, most people play random for their faction, and frequently end up with good vs good, evil vs evil, and mirror matches. If you make Catapults do no damage to gates or walls, then Rohan essentially cannot win against another good faction with walls. Their onagers are already so weak its pathetic, and this idea would make Rohan completely useless in siege.
Next, what happens when Gondor or Dwarves have filled all of their wall plots with trebuchets/catapults? If only Mines and Trolls can damage these structures, all it takes is a couple of fire archers on the walls and your castle is unbreachable.
Finally, the notion that Catapults would do no damage to gates or walls is unrealistic and, sorry to be blunt, stupid. Hurling a huge chunk of rock, or a fireball, at a structure is going to do damage, whether its made of wood, stone, or metal. Also, walls are not particularly expensive to rebuild in the current version, and evil armies take a while to re-assemble, so all you have to do is survive the siege, then you can attack the enemy while they are weak and rebuild your castle at the same time.
The 1 idea that I saw here that is really good IMO is to have Rams be really weak against everything if they are alone, but gain a passive armor buff if surrounded by troops. This would simulate cover from archers very well, and would make a lot more sense than having rams that are immune to arrows, like the current version.
Final last note, for those of you suffering from ram spam or ram rush, all you need is 2 battalions of Archers equipped with Fire Arrows, and you can survive any attack made by rams.