The reason I left it untouched is because of how easy bombard towers are to defend. They kill 95% of the units instantly. Put a single guard next to it that kills rams, and you have a decent combo against rams+escort already. Wars around BBT are already hard enough as they are and civs with BBT should not have an easier time for that.
It's really more of a balance thing rather than realism. If we went for realism then a well aimed cannon ball could probably break a ram's structure in one shot :P
Well I agree with the assessment you made, the point indeed is that fighting mass BBTs with a army around them is indeed hard enough