I would think it is just a case of orthogonality, of every combination being available. The 'S' is just so you know the x bit is not set. Likewise, you can have rwx bits all off - it isn't particularly useful, but you can do it.
Conceivably there might be a scenario where you don't want the owner to accidentally execute the setuid file. This doesn't prevent group class users and other users executing it, assuming the other execute bits are on. But it would be an obscure case.
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