It had 2 effects, it showed that there were still people who existed outside the scope of the current world and were still a part of the old world, a way of contrasting what once was with what is, and it showed a group dynamic with Michonne in it. Specifically, it showed her interacting with a troubling aspect of being in such situations - if one of the group is actively threatening the lives of the group, do you try to restrain him (potentially at the cost health to the restrainer, or to the entire group should they get free) or should you kill them outright? It's a common thought experiment for how people take responsibility for the lives of those around them. example, in three stages:
1 - your standing atop a hill and there is a train coming a long ways away. There is a break in the tracks where the train will have to go down one track or another. On track (a) there is one man, asleep on the tracks. On track (b) there is a group of 20 men working in a gorge where the track twists around a blind curve. Neither group will be able to escape the train - the sleeping man in drunk and the men behind the blind curve have no where to go. You have a switch in front of you that will set the train down one or the other. The train will kill someone no matter what, one man or twenty. are you willing to set it to track (a)?
This question has a lot of people saying yes. Your responsible for deciding the fate of the men, but at a distance. The level of responsibility is minimal.
2 - your standing on top of a archway that is above a bridge, underneath you is a train track. From your heigth you can see into the distance behind you the same gorge in example 1. There are 20 men who are in a gorge that the tracks run through, around a blind curve who have no way to escape. A train is coming in the distance. There is another man right next to you. You know that if you shove him off the top of the archway onto the tracks that the train will stop because the conductor will see him. You'll save 20 mens lives, but you have to sacrifice 1. Are you willing to push the man?
This example has the same odds, you are still responsible for choosing who dies, but the responsibility is direct. Most say no.
3 - Your in nazi germany. There are nazi's searching for your party of 20 men woman and children. your hidden in a basement with wood slats covering you. A baby in the group starts crying. You know that if the baby continues to cry it will give you and your group away, but to silence it you have to silence it completely, meaning suffocation. Are you willing to save 20 men, women and children by suffocating the baby?
This example has direct responsibility and it crosses social and personal moires. Almost all say no, yet it is an example that has happened in real life.
The man in the cabin was Michonnes test of these 3 scenarios. She has presented a willingness to go as far as number two. Rick has shown only number 1, if that. He shoved the guy into a yard full of zombies to save his group - but he didn't pull the trigger on an unarmed man. While you could say that him killing the guy in the prison was equivelant, it's not. That was a direct threat to himself, not an overall group threat. It would have likely lead to that, but in that instance it was "I'm going to kill you, right now, with my bare hands", not "I'll kill all of you by unleashing a horde of zombies on you, but I'm not going to kill you directly".
It represents a kind of litmus test for willingness to kill, and under what situations. If you ever want an excellent example of how people slowly build up to the three levels of violence read the lord of the flies. One of the best books ever written, even more so when looking into the visceral experience of building a tolerance to commiting and witnessing violence. So far the show has not reached level 3, and it might not given that it is on tv.