If you look closely at the functionality that's been re-implemented so far, you'll see that the majority of things are following the rule mentioned above. The main advantage is that you can tell straight away what's an ID and what's a class. I think using dashes for framework classes and underscores for other classes and ids as a 'rule' makes things even more unnecessarily complex.

If we only want to use 1, we'd have to use dashes for everything to be in line with things like bootstrap. I'd be fine with that as well.