There's no substitute for experience. I did several years instructing on singles and light twins VFR and IFR, aerobatics instructing and aerial photography.
Aside from handling skills it gives you captaincy skills from day one. You have no one else to help you or rely on except yourself when a difficult situation crops up or a go/no go decision has to be made. Also you often don't have the system redundancy as in pax jets with GA aircraft often being quite old so comms failures, flaps locked, engine problems and other types of situations happened from time to time. Sometimes coupled with poor weather.
So to say 1500 hours instructing in the circuit is useless is a bit naive really.
When I moved into the airlines and it came up in conversation with Captains they always said that they saw something was lacking from the guys coming directly from flight school in comparison.