The point is still very valid. You can't add a math coprocessor to the 486SX.

The 487SX is nothing more than a polite fiction to allow consumers (and the computer shops selling to them) to continue their existing habits of buying FPU-less systems, (because most people didn't need FPUs) safe in the knowledge they could buy the upgrade if they ever needed it.

It actually cost the motherboard vendors quite a bit more to wire up the second socket, so obviously there was demand for the flexibility.