but then you have to do like CRYSIS and make chunks of trees that are broken to simulate what CSG does, and fake stuff like that which complicates development more than actually have the real CSG commands.
Current engines are faking CSG right because doing CSG on large portions of a level and/or with animated meshes actually complicates development much more at this moment in time (unless you're not the one coding this feature of course). Some developers who were willing to implement it avoided to do so in the end (such as Bohemia Interactive in ARMA 2). Other ones implemented it in a simplistic way and only on static meshes (such as in Red Faction, quite limited, and Silent Storm, which wasn't very performing but was turn based anyway).
On a side note, you also have to adapt the AI to dynamic environments, and that's another reason why developers are carefully with the idea.
It is a feature which is getting more attention now (which will still grow in the future), but in most scenarios it can be more efficient just to fake it.
PS: Hey Qimmer, the post processing blur ruined the screenshot for me! :)