Here's a hypothetical on the whole "stay in your group" idea. Surely by voluntarily stopping, you cede your position in a group, and if the next bike passes you, you become part of that group? Urán et al were not forced to stop by conditions on the road (i.e. a crash, blocked road, crossing etc.) but chose to stop. If Quintana/Izagirre/Rolland/Hesjedal had to stop there because Urán et al stopped, what happens if the next group on the road then arrives? Do they have to stop at the summit and wait for the previous group to set off again so they don't overtake them? Do they join that group? Do they overtake the group and Quintana have to stand there like a lemon at the summit?
It's nigh on impossible to police so many riders and to say "you must stop because he stopped" should only apply if it is for unavoidable reasons. Quintana and Rolland were still behind a motorbike, so they were still heeding the descending warnings.
Poorly managed, hell yes. But it's not Quintana's fault and by expecting him to stop and stand around because Urán did, or to cede the advantage he had because it was gained unfairly despite it being the other guys' choice to cede the time, you're getting into the Andy Schleck realm of "descents should not be raced". In that case the race leader can effectively kill the race until the final climb and ride down like Ivan Basso's grandmother stopping every five minutes for a break, because everybody else has to wait with him. Obviously that's an unrealistic, extreme example, but still, we need to consider that there was no obligation to stop. Even if the race was neutralised, they could have safely guided Quintana's group down (after all, descending is safer in smaller groups as you can pick lines better) and then had them sit up at the bottom. They didn't, therefore Nairo's in his rights to capitalise.