Prep sinks certainly serve a function/purpose.

Dishes go in the cleanup sink, that is its function/purpose.

It's fine if you don't, but lots of people cook with their spouses.

In reality, I don't really care if you (or a client) would prefer not to have a prep sink. In the end, it's your (or their) kitchen). I'll simply point out the advantages and show a design the minimizes their concerns to give the best of both worlds. If they reject the idea, that's part of the job.