For me as being on the 121 Captain chair. Having an F/O with ATP has 2 reasons:

1) Pay your dues buddy get to that ATP 1,500 hours finish line before flying 121. Any flying as Captain makes you a better pilot, because the decision was made by you no matter the scenario. You did not killed yourself or another. So flying 1,500 as CFI it shows me a better candidate to be on my side on an Engine Fire than a 450 hours jet jockey...
2) To my understanding ATP means AIRLINE Transport Pilot and both the PIC and SIC are AIRLINE pilots so why the difference of having an F/O without an ATP?

I flew with those 250 F/O's and the day is longer and more stressful. Is like being a training captain without being paid xtra for that...And Sorry if I am AGAINST this rich lowtimers that make my day more dangerous 95% of the time...