Bringing a side piece around your friends can be touchy. Have to sometimes worry about things getting weird if your friend falls for her or vice versa. Personally, I also find it hard to control the frame of 'we are just kicking it' when you are around other friends as well. Someone always makes a joke about how you two are a great couple or how you hung out with another girl last week. It is a lot more variables that can fvck up your game when you introduce a lot of noise into the relationship.
Also though, if you treat a girl like a girlfriend, she is going to want be your girlfriend. I don't agree with the idea that you have to treat girls you are fvcking like sh!t, but I just want to remind you that if you do hang out with them a lot like a girlfriend, you have to actively remind them that they are not your girlfriend and you are not looking for a relationship.