I worked as a line guy for a year. The company gave interview to the line guys who qualified. They didn't give jobs to all of us. There were a few hired that you could categorize as dangerous, however even after 700 hours with the company I think they're still more dangerous than some of the new guys coming in with low time. It really depends on the person, but I agree that 300TT is relatively low.
To be quite honest, we did start alternating legs from the beginning. Personally, I think the harder of the two roles early on was being the PNF. Acting as PF was easy. It was just like flying a Beech Duchess. Except the airspeed was faster and you had your PNF doing all the dirty work.
But this is all stuff for another thread I suppose