Unpainted plastic looks like, well, unpainted plastic. There's an oiliness to the finish that even when polished to a gleam in cars looks like plastic. For military vehicles, you almost always want a flat finish, not glossy as with unpainted plastic. Also, if you have decals, they will not adhere well to plastic. There are also often slight variations and swirls in plastic, most notable in silver. Glue will be very visible as well, even iif well sanded. Filler? Fogeddabahdit! How you gonna blend that? Different parts may also have variations in shade. When cutting from the sprue, you'll have to dress the piece, sanding etc. That will show up s different shade and/or texture.
Yeah, if it's inside and buttoned up and there's no viewing access, don't bother. But, as a wiser modeler than I once told me, "If there's any chance someone's going to see that part, paint it."