As I said above, I don't think that a coaches challenge would have made any difference in the Quins game.
However, I'm not entirely against the idea of each team having a number of challenges that they can use throughout the game, but there's gotta be some changes to the existing use of the TMO and how the challenges could be used.
Ultimately though, the more i think about the idea of Challenges, the more i think it will cause more trouble than it's worth.
I think it's fair to use the TMO for checking scores (i.e. is it a try, did the kick actually go through the posts), and for foul play.
However, maybe the TMO should only be allowed to intervene off his own back to check for foul play (I believe that was the initial rule)
Anything else is up to the teams to challenge.
You'd almost certainly end up with tactical challenges where a team are under pressure so the coach challenges just to get a break in play and let his players have a rest.... that would be bad, but would definitely happen.
The idea of letting play continue until the next stoppage before reviewing a challenge also isn't ideal, because that could easily be a few minutes, maybe even 5 minutes.... and that seems too far to rewind to review a challenge for what could be an illegal rip at a ruck, or an offside, knock on etc.
It seems like a total minefield.
Instead, as others have said, get the existing rules correct before trying to fuck things up again