Ballistas can be useful, but when you have access to Ornagers there isn't much point in having them.

They are more designed around attacking wooden defences rather than full on stone ones... But even so they do have their uses.

Once when attacking a city with Stone walls I was able to pick off the defenders on the walls with a couple of Ballistas and skewer the enemy captain! (quite a sight seeing him flying backwards off the wall)

Personally I like them due to their accuracy and find they make a great companion to a few ornagers... Afterall you never know who they are going to skewer!!