Performance wise, having 2 materials on one object is equivalent to have 2 "splitted" objects with 1 material. Usually i only use this for easier maintenance, as editing several objects in a 3d software could be a bit more of work if they're meant to be "welded" Unity can batch together objects with the same material…
if your objects already come with per vertex shading then the hit comes at the seams/hard edge as that requires another set of verts at the seam in the engine (which would also occur with a mapping seam btw) so the overall hit is usually quite small but it's worthy of consideration.