"I have to say that a pilot with 1500 hours of flying from XXX to YYY and isn't materially any more valuable to an airline than one who's done it for 500. There's only so much you can learn in a CRJ or Embraer."

You will get from the experience what you put into it. If you choose to be a $#!tty instructor, you aren't going to learn a darn thing while doing it. By your logic, everyone should be ready for upgrade with 500 in type - are you ready to defend 800 hour Part 121 PIC's?

Experience is not only measured in hours, but also years. You learn things flying day in, day out over a sustained period of time that you will draw on for the rest of your career. Not discounting hour, but if you could build 1500 hours in a summer (not possible I realize), do you think you'd be ready for the left seat having never de-iced or operated in icing conditions? I realize that most SE trainers don't deal with the previous two problems, but what about analyzing if you can conduct an IFR training flight in a non-known icing aircraft in the midwest in January? Would you learn anything from that exercise? Would you be developing judgement? Would you be evaluating options and alternatives if you were to go? Do I need to continue?