Some obvious philosophical questions.
Can a training FO become a captain overnight? Of course not. So how can someone with no command experience help develop new captains? How can someone with no command experience judge the actions of an experienced captain?
Sure, if it's just a question of grading a hand flown single engine ILS, then I don't mind to be trained by an FO. But now that training leans more towards the LOFTS scenario's (with all inherent vagaries and grey areas) it is a valid question wether and FO with no decision making experience is the most appropriate trainer on the day.
I'm sure a training FO will do a good job 9 out 10 times. But there will be areas where he lacks experience or dare I say, credibility.