Well first of all, I do use trebuchets and siege units in general to take cities. I don't see why anyone wouldn't use them. And I do think that the turtle ship is even stronger than England's ship of the line. Of course the fatal flaw is that it can't enter deep water. But I do agree that the UU and UB for Babylon are not that big of a deal.
I stopped using siege units alltogether until I got cannons. And even then, I mostly wait for Artillery. In my oppinion, they are to slow... The problem is, that a bunch of Comps / Xbows can do the same then a siege unit, without the problems (setup time).