I work in banking, though my hours are decidedly not
that bad. But then again, I don't work on Wall Street for any of the bulge bracket firms. Sometimes I have to work 80 hours, sometimes I can work 45. It just depends on what's going on. Anyways, yes, those consistently working 80-100 hour weeks would have a hard time with extracurriculars.
Specifically, my firm brings the volunteer opportunities to us. They don't do it to make our resumes look better, but they understand that we can't commit to regular volunteer activites so they build in the time for us. Once a week we tutor 3rd/4th graders after school, but they come to our office. We are assigned the same student for the whole school year. This is my fourth year tutoring, and it's lots of fun, but particularly helpful to the kids who need help with homework or improving their math or reading skills. I know that many big firms are involved in the same program or do something similar.
As for other activites, I do what I can when I know I'll have the time. I don't think that one should depend on their firm to provide the necessary EC's to get into business school. But I know that many of my banking colleagues are all about doing only the activities that will get them into school. Personally, I'm not involved in other EC's because it would get me into school, I'm involved in them because I enjoy them. And because I enjoy them, I make time for them.