Many things are done out of ignorance.

If one uses an optoisolator when a simple OC-relay-driver, e.g. '75451, etc, would do the job, then, yes, it's worthless.

You can misapply nearly any circuit you wish, BUT, if you want isolation, you can get it with a properly applied optoisolator. The key, of course, is to have no common power or ground connection that affects both the input and output circuit. If they have a common GND, then a high-voltage OC driver e.g. 7416 should do just fine.

Lots of optoisolators are used because the designer was uncomfortable with components with fewer than eight leads.

RE