If you see three guys running to a specific part of the map, then you should probably join them. Chances are they know what they are doing, and even if they don't, you have a better chance of winning if you stick with your team.
...or they're three PUGs who are all running into a turret and doing a combo of being trashed by the turret and sliced by phantoms with the obligatory marathon-revive.
Or sometimes you're the only infiltrator and you're trying to be invisible and get the device objectives, and the rest of the PUGs decide to bring all the agro to you. Especially funny when that puggernaut lumbers over to drop his hex shield on you which is commendable but by the time he gets there (with the two banshees in his wake) there's only 1 second left anyway.
I can't even tell you all the times that (miraculously) my PUGs actually all got into the hack circle only to get overwhelmed by Praetorians and Scions and nuke-zombies, and even when I revive the hell out of them and turn the mic on just to say "Guys, we have 3 minutes left, everybody run to the other side of the map for a while!" only to have them stay in there and get massacred while I wait in safe cover, then I have to kite the enemies over and get the hack done myself.
What I'm trying to say is strategy isn't a monolith and, as they say, no battle plan survives contact with the enemy.