I don't think casualties matter much, losing 1 treb is almost the same as losing 2 catapults. Given that most of the fighting is not against cities anyways(or at least the problem is not taking out well fortified cities, that you can do just as well with cats as with trebs), and cats are strictly better than trebs in the field, i hardly see a reason to build trebs(especially since cats both cost less and give more colleteral damage).
Check this game out.
http://forums.civfanatics.com/showthread.php?t=298806 In it I am using just Trebs,LB,Pikes and x-bows to take cities. At the point of the last post, I had taken 4 cities defended by LB's,x-bows and maces with the loss of just ONE treb. One unit to take 4 cities is a pretty good hammer investment. I purposefully waited until the AI had units from the same era so that I was demonstrating the power of siege rather than the power of a tech advantage. If I had used cats to soften up the LB's I am quite sure I would have sustained much higher losses. Trebs are significantly stronger than cats when attacking a city. They also bombard defenses at a much faster rate. I now have a stack of about 10+ CR3 trebs. I do not forsee losing many of them until the AI has muskets.
Cats are better in the open field and should get the barrage promotion (which was argued to death in another thread).
To get back on topic. I very rarely build knights for the offensive. As a dedicated seige unit warmonger, it really doesn't matter what unit I use to attack a city with after the siege assaults. The defenders are all redlined at15% strength. As stack defenders they are greatly overpriced. If you are concerned about x-bows, then bring some of your own. Personally a few cover x-bows are just fine for stack defense. Even if facing another cover x-bow on flat ground they have even odds. And most importantly your precious, and IMHO highly overrated, CR melee units will not be selected as the defender. The 2 moves are wasted due to the slow speeds of the units required to protect them.
I do however build them for defensive purposes. I keep a stack of them in the homeland as well as a couple small stacks of cats. When the SOD is next to a city, i attack it with the cats to weaken the spears/pikes and them the flanking2 knights. It wipes out most of the siege then I gleefully watch the AI suicide and an entire SOD onto the pikes and LB's.
The last reason I go light on the mounted units is the limited upgrade path. Once the AI has rifling, you're hosed. You are stuck paying maintanance on your mounted units until advanced flight.
One of the few gripes I have about civ is the minmal army vs army fighting in the open field. Medievil wars were not usually fought as sieges. Great armies would meet and wars were determined on the battle field where knights were unmatched. When a city was attacked, it wasn't the titled guys in expensive armor that attacked the walls. It was masses of poor shmucks that couldn't afford horses.