Ok, here's another opinion on trebuchets (or artillery type units in general):
I think, most of the time you can get by without them. I've done fast Conquest and Domination games on Emperor/Demigod without building a single bombardment unit. This is true especially for a war under Republic.
If you are using 2-movement attackers (horsemen, knights, cavaly) the losses are lower than when using 1-movement attackers (swordsmen, MI). Therefore it is not as important to "soften up" the defense. It is better to loose a Knight once in a while and having to replace it, than to keep a huge stack of trebuchets ready, which will only seldomly be useful. A stack of 7 Knights (490 shields) will be much more effective than a stack of 7 MI + 7 trebuchets (490 shields), simply because it moves twice as fast and takes only half the unit upkeep. A dead knight can be replaced quickly, if you have roads to the front line.
Also, slow-moving stacks will spent a significant amount of time travelling instead of fighting, and this is where they eat your money without giving any benefit.
In my opinion, building artillery type units is necessary/useful only in the following situations (basically the same situations that call for a "combined arms" approach instead of the "single unit type" stack approach):
- You are technologically backward, e.g in a Demigod/Deity game. (If your Knights are meeting Musketmen, you definitely want artillery support...)
- You don't have horses. (And no fast UU like the Gallic Swordsman.) Or you don't have iron and need to rely on Longbows.
- The AI has many towns on hills or with city walls. (Even spearmen in a wall-protected hilltown is a very tough nut for Knights...) In this case, two separate armies might be a good idea: one consisting of lets say 10 trebs, 2 pikes and 6 MI to crack open the tough nuts, and another Knight-only stack to rush forward and take the lightly defended grassland towns.)
Things become slightly different, once railroads are available. Artillery proper (or cannons, if you don't have rubber) combined with railroads are deadly, when it comes to throw back an enemy SOD that has broken into your territory... And even in the offensive they are often useful, because they can use railway to keep up with the advancing Cav/tanks and they can shoot from the distance. (So they don't need to loose much time for advancing all the way up to the enemy town. If the enemy town didn't culture-expand yet, Artillery can hit right on the first turn, and then the tanks capture it. If the enemy town got it's first expansion (and you can't use a combat settler), tanks and artillery move one tile into enemy territory during the first turn, and in the second turn the town can be taken.)