I could count on one hand (about 5%) the bands I've worked with that actually stayed with the song list.
As others have said, it is a guideline - but you need to adapt to the audiance. I've been in "Disco" bands that dumped the list and played metal when our agent booked us in a biker bar and I've been in jazz groups that pulled country tunes out of a hat when needed. I've been in plenty of bands that don't even use a song list.
If you are a show band or a "concert headliner" then you need to stick to a list to coordinate with lighting, props, etc. - otherwise be prepared to adapt.
As an aside, I book a lot of gigs where I put together "pick-up bands" where I hire the best players available for a given giig (we've all heard of the A list, B list, etc. players).
Rather than having a song list, we just rotate. I'll pick a song I want to sing call out the key, then the next song the bass player calls out a song, then the guitar player, etc. and we rotate all night. It keeps everyone on thier toes and can be alot of fun (except for the occasional train wreck).
Two years ago I booked a 3 week stay (14 gigs) at a club and had a different line up every night. Naturally you need some fearless, seasoned players to pull that off.