In the opening 20 minutes of The Hangover, the script quietly establishes that Doug (Justin Bartha) is the only thing that ties together the three other characters together. He's the peacekeeper and the normal friend, and it's clear that without him the other three guys would have nothing in common. The script then removes Doug from the picture entirely, leaving the other three characters to deal with each other. While most movies are forced to continually up the ante in order to get the characters yelling comically at each other, The Hangover could let them start screaming from the moment they wake up with the titular hangover. Then, once Doug rejoins the group, they aren't forced to have a "y'know, I'm just glad we're friends/best friends/husband and wife/partners/brothers/Wayans brothers/literally birds of a feather/almost done with this Michael Bay movie" scene. The movie could just keep right on moving to the resolution.
In the inevitable sequel, I guarantee that this element will no longer be there. Either they will remove Bartha entirely from the movie, or he will remain with the group the entire movie, spoiling the chemistry. A much better solution would be if they somehow managed to once again get separated from Bartha for the majority of the film, however they needed to do that ("alright, guys, I'll see you in New York City in 24 hours for the wedding/funeral/bat mitvah/birth of my child/briss/disarming of the nuclear bomb. Don't be late."). Unfortunately, I'm betting that won't happen.