This is a tricky subject.
Cycling has a long tradition of having gentlemen riders, who had honor, courtesy for each other, and respect for the race.
So the idea to wait for the leader when something happens out of his control, is a very old one and based on this tradition.
On the other hand, there are many sports where a mechanical failure is actually the best time to attack, for example: all motorsports.
The only time when they can't attack is when the yellow flag is out (jury's decision).
Mechanicals and crashes are part of the race, and should be treated as such.
When a racecar driver has a mechanical or a crash, he isn't angry that the others didn't wait, and it's fair game for everyone.
Many times, the overall champion is the one who had the least problems, not the strongest or the fastest.
The difference from motorsports to cycling, is that in the former, they are always on the attack.
In cycling I think we need a change in mentality, shifting the anger from the opponents who attacked, to the mechanical or the crash itself.
The ability to accept this is going to take a while to sink in, but I believe it'll be good for the sport in the long term.
Mechanics and bike builders need to be even more relevant to the success of the teams.
With that being said, I actually think there's a time when the bunch can wait for a rider who had a problem, far from the point where decisions are made, probably before the last 50km, and not on a climb or cobble, not pursuing a chase, etc.
This might simply be called "fair play", like what happens in soccer when a player is injured and the other team throws the ball out.
That team will get the ball back on the side throw.
But this only happens if they're not attacking, if they are, the injured player will have to wait for the play to finish or for the referee to stop the game.
TL;DR - My answer is "Whole pack together at a non-critical point" just because of fair play, not any unwritten rule.