Cook had already been repulsed at the Rosebud. Custer divided his force into three columns, himself, one column under Reno, and Benteen in the rear.
Reno's colunm was being mauled even before Custer rode into the ambush. Benteen had been ordered at the last minute to catch up with Custer. When Benteen caught up with Reno, he decided to support a demoralized Reno, who had lost half of his men, instead of trying to meet Custer. Also some of the Native Americans who had been attacking Reno went off to finish off Custer.
As it was Benteen and Reno were barely able to survive. If Benteen had obeyed orders and tried to meet up with Custer, he would almost certainly been masssacred. And if Benteen had not been there to shore up Reno his column would also have been wiped out. All three columns couild well have been wiped out, not just Custer's one.