We work for a very man-and-boy outfit PENKO, the background of the vast majority of current FOs and the usual shock/amazement from the left seat when you didn't just graduate from [OAA/CTC/FTE] last week is a testament to that. Yes the gradient is very shallow and flight decks are usually pleasant, but usual the assumption (as confirmed by past OM-B/C changes) is that the FO is inexperienced.
Now, about training, one of my favourite instructors during my initial TR was a FO and came up with lots of uniquely good advice other more senior Captains didn't think to share, maybe because day one wasn't so long ago so he still remembered some of the gotchas?
I don't think one needs command experience to explain someone how to deal with technical failures even when they affect airworthiness. If said pilot can successfully pass an instructor or examiner course then they have proved themselves capable and the quantity of stripes decorating their uniform should be considered irrelevant. As many have said already the FO may well have been in a teaching and/or decision making position in a past life - maybe not in an aeroplane - but then the non technical decisions aren't always to do with the aeroplane anyway, it's the bigger picture that counts isn't it?
All this is quite academic/utopian anyway... Much as I would like to think that the industry is/will slowly open up to the fact that some FOs may be qualified and inspiring instructors I sadly believe the reality is that training FOs will become more widespread for one single reason: cost savings.