I imagine that a lot of the musicians who play at events like that are doing it in the hope that the exposure will someday get them more work or start their career.

It's common practice in politics and the legal profession, where graduates can work for as long as two years with no pay to get on the bottom rung of a gold-plated ladder. The result is that those professions are only open to people with wealthy parents.

Mumford and Sons doubtless don't see a problem.