I meant that in many cases you might have a hard edge in between metal and non metal parts. It could be the same pbr shader but you have to split UV islands along hard edges anyway. And if you don't want to do it for some reason the real time engine would split it nevertheless .
one down side splitting uv, hard edges is it can increase the memory requirement of the model in game as it adds additional vertices (an in game cube for example would require 24 "verts" to display (not the conventional 8, though that is pretty much a worst case scenario)
Separating UVs still might help if you having too ugly "metalic" halo, especially on low res textures. If the thing have hard edges it get's lots of UV splitting anyway. Although I personally sometimes just do rusty/dusty/dirty parts also metal, non metal or half metal to kill damn halos instead of messing with UV