The 15 additional minutes gives me a bit more time between the headliner change-overs (which can be an absolute bitch). The show needs to be over at 1:30am.
I will say that all bands know there is a 30 minute allowance for change overs. If a band plugs-in and is ready in 15 minutes, then they get to add that 15 minutes on to their set. You can go on early, but you cannot stay late in other words.
Glenn H.