I believe set times should be posted in good time before an event, no questions asked. As a venue, promoter or band you will never know the individual circumstances or commitments of the guests attending your gig, who after all are already supporting you by paying for a ticket. It’s a personal choice whether they decide to watch the whole gig, part of it, or as someone has said above, attend to watch the support band and then leave (this is more common than it might seem!), it’s almost as if they are saying you are to be held hostage all night if you are attending a gig. That’s not really fair, and quite hypocritical, most headline bands, and sometimes promoters themselves aren’t even around to watch the support bands.
Personally a lot of times I find set times useful as re-assurance that I can make the last train home.
Set times are commonplace for people attending a festival in order to schedule performances around their day, so why not gigs?