Should have ended about 5 minutes earlier

yeah.. Roger was a loser.

You know, I think I agree with you.

While I agree the final scene was a bit odd and maybe a little forced, I think the point was to show that the night together changed not only Nick, but Roger as well. Notice when Roger is talking to the boys how he tells them to be themselves, etc., and compare that to his opening instructions to Nick (check out chicks in the window, etc).

Only if you see the movie as some sort of coming of age story. If you see it as a character journey for both Nick and Roger and as a relationship, the last few minutes make perfect sense. Roger was in a very bad place that night when Nick dropped on him. He was bitter and rejected taking it out on every woman he met.
When Nick comes to him, he sees it as a second chance to change the game, to prevent Nick from making the same "mistakes" he thinks he made, being too naive as regards to the position of men in relationships.
He realizes that's what he is doing before the night ends and decides to give it another try, from a different perspective, at the end. His goal is still the same: prevent Nick from making the mistakes he did, but this time, he perceives the mistake as being a different sort. His cynicism and shallow perception of women and relationships.

For every lie I unlearn I learn something new - Ani Difranco

It's an interesting idea, but it would really change the tone of the movie. It would be kind of dark to end it there. The scene as is injects a bit of light hearted fun into things, making the film a bit more positive.