Well, unless its along a UV border anyway, then there isn't any performance hit as the verts are already doubled there. I always run a script that hardens edges along UV borders, as its good for a lot of other reasons(poorly synced shaders, compression, extracting detain with CB, etc).
Probably when I agreed with you. =P There are two valid points to be made here: A. Using a faceted mesh/hard edges on every face is a bad idea. Both you and I have made this point. B. Using hard edges along your uv boarders on a normal, well optimized mesh has no drawbacks. - You keep bringing up A whenever someone brings…
If your hard edge is along a uv border, it doesn't get split "again" the "real" vert count of your model in a game engine will always match the vertex count of your uvs, if not, your exporter is doing something funky. There is a max script somewhere that will count your actual vert usage. I mean, of course it is a terrible…