RFG,
Your idea of having 300 hours in command sounds good, but so many instructors use their rating as a means to obtain some hours so they can move on to charter or similar otherwise they will find it hard to land a charter job with a bare CPL.
It's a catch 22 situation, it's very difficult to get a charter job with a bare CPL so the easiest way to go is get the instructor rating and move on. If a requirement for 300 hours came in then it would be tough to get that initial job, or there is the option of paying for private flying but I don't think many of us can afford to pay for another 220 hours after finishing our CPL's. If they did secure some charter work then would they honestly want to change to instructing after having the experience of all that stick time?
Just my opinion.
Cheers
CC