A few years back I stumbled into a gaming group and thought it was pretty much heaven. The group was pretty awesome, everyone clicked, and we were all involved in a very awesome and intense game that is still fondly talked about all the time by the various members. It is effectively the default "L5R game" that we talk about when the system comes up, sometimes diverting to it even when talking about current games.
Now, at the time when I realized how strong that group was I figured I'd be happy to be with the group all the time going forward. It's not a bad thing, really. But times change, people change, and for all the "oh poor you" that this will get, every group being the same when you have as many simultaneous games going as we have can be a bad thing.
How so? Well, think of your favorite author, director, or screen writer. You can only consume so many stories by them before you start to notice the similar trends. A minor character in story A is a major character in story B and the villain in story C. Allison always has to play the finesse based rogue and Timmy is always looking for the mechanical edge to steal the spotlight in combat scenarios. Basically, it starts to get stale.
The awesome thing about Gaming as a hobby though is that you likely won't notice this in most groups. Most people have one game that they're in, and those go so long and so slowly that you could go years before the recurring trends start popping up.
Still, this isn't meant to be a complaint about my core group. The group is still awesome, despite some social changes that have happened. But I'm also glad for the games that have different players. The Wednesday Shadowrun game has one player that isn't in most of the other games. The Sunday Star Wars game has two. And the other games have enough of a shuffle up that even with a lot of the group being the same core people there is variety. One new person provides plenty of stimulus to break the stalemate. People have to react to that different element, and that alone can get people riffing again.
Variety, it isn't something to be kept at bay pointlessly. :)