I really can't agree that as a yardstick of professionalism... For some gigs/bands, sure... but personally, most of the highest level wedding/casual/party leaders I ever worked for - never, ever played from pre-determined sets. They were literal masters at reading the room, the dance floor, coordinating seamlessly with the banquet manager.... so the dance floor was always full when that was desired, that band breaks always coincided with food being served - always making sure the band was playing music that fit with that particular crowd on the particular day - rather then just forcing whatever the band had pre-determined to play upon them. It was pretty impressive - and you never knew how it was going to be... Some gigs would be pretty high energy throughout - while others would only occasionally get out of cocktail hour mode.
My point - there's lots of different approaches to doing gigs.... So while I get a leader being unprepared would be unprofessional. Just pointing out that in some professional circles, not preparing set lists would not be deemed that at all. (Actually on those gigs - asking for a setlist would be a red flag suggesting that the player may be the one not up to the gig.) There's just lots of different levels and niches in the world of professional music playing.