My two cents worth:
A single end tenoner can be a useful machine to have. A lot depends on what you would want out of it. I agree that for standard cope and stick cabinet door a shaper with a jig or sliding table would be more feasible than a SET, better yet outsource your doors when possible. Cutting simple tenons on a shaper is a breeze with the right setup. A dedicated machine makes sense if you are cutting longer tenons or tenons with offset shoulders. I wish I still had a small single end machine some times.
Another consideration is the cost of tooling. cope cutters for deep reach cuts can get expensive quick so can large diameter cutters for a shaper. It all boils down to what you can get out of owning a specialized machine.
CW
