A verbal agreement will still form a contract. At it's most basic, a contract is defined as a service or product given in return for a benefit. It could be argued that the benefit received by the band could be publicity, or free drinks etc, but there is still an implied benefit. Likewise, there is an implied benefit for the pub or club, whereby they can receive a financial benefit from increased bar takings.
So, you could [i]probably[/i] win a case if it came to that, although you'd have to be prepared to fight for it.