Bellsey: flexible or not, only a .ztl allows for easy changes to be made, as well as stuff like layers and subdivision steps. It's nothing to do with exporting into the game, it's so the client can adjust things if need be. You'd provide them with an obj/dae/fbx of the low and highpoly as well, since those can be loaded…
My point was that ztl, isn't very flexible and only works with Zbrush, nothing else. Though to be fair its the same for .mud or any other scene file format I'm not rubbishing formats like .mud/.ztl, like any other specific software scene format are great at what they do, but not so good when it comes to pushing data…
well, when they've asked for .ztl files, i'm assuming it's so that if they need to reposition things to make baking easier they can do (i tend to have my low poly w/ UV's as a subtool within the .ztl file so you can use transpose master to move everything about all at once). or if they find that they might want a certain…