When you roll with even just one person over and over again, it is clear that you are able to defend against his/her moves better over time.
The benefit of this is that when you fight someone with a similar style in the future, you are able to defend against their moves as well.
The problem is that if you are limited to a very small group of people to roll with, you will not be able to defend against people that roll differently very well.
Styles make fights whether it be BJJ or MMA.
This is one of the reasons why competitors either invite athletes from all over, or go and visit their gyms. Variation (and lots of it) is key to becoming well rounded.
Now a great match between Chris Moriarty and Shaun Smith