Without knowing the welding module at all, I am guessing that the spot weld feature needs to have a point on surface as the locating feature to be able to infer the location of the spot weld and also the surface being welded. Perhaps you can use your point but you also specify the spot weld surface?
Kind of similar to how hole tool will use a point on surface to make a hole with just that one reference, but if you use other point types, it will require you to also specify a 2nd reference - , namely the surface to be drilled (even though the point is already on that surface)
