Yeah... that, "Hey buddy.. can you put us up next, we have to leave." BS is just that. BS.
Don't fall for it. The answer is always, 100% no, because as soon as you get them up to sing, they are there, 100% of the time for another hour, and some will be total jerks to you knowing they got one over on you.
When I was new, I fell for a sob story a couple of times, but after I got burned a few time, I learned my lesson.
Be firm, be friendly, and be consistent.
The other one is, "Hey my friend/ boyfriend/ uncle/ sister/ life partner/ whatever is the lead singer in a band, he/ she/ it is going to rock this place!"
This, my friends is a tell, and its telling you this drunk dufus is going to SSSSSSUCK!
Don't fall for that one either.
I say, "Hey, that's great, and I really am excited to here them sing.. but to be fair to everyone, everyone has to wait their turn." Smile, nod your head and move on.