The core activity of a band is making music. The bottom line can be helped a lot by marketing. Other roles may be very important to allow the musicians to focus. Sometimes musicians need to be kept sober or made to practice more, with which others can help. So other people can make value in a band and deserve to get paid. But take away the musicians, and you have absolutely nothing. Plenty of people have made good music without lots of managers over them. In that specific sense, musicians are the most important part of a band.