The reason why ballistas are allowed is simple: Other art units sometimes hit the target, sometimes not... it depends on luck, trebuchets, bombards etc have a very poor accuracy while ballistas hit their targets in the majority of cases (~99,9%).

The reason why rockets are allowed is, as pariya said, they costs 1100 Florins and have 3 volleys.

Here is the link where it started a while ago...
http://www.clancommunityshield.net/t...hp?threadid=75




Now pary... back in your cage!