1. You learn a lot
If your man/woman is checking out your hot friends and is very flirty- you learn a lot. Not only in terms of their loyalty, but also in terms of their boundaries and respect towards you. It is important to see your partner in my situations, including in surroundings that may provoke sexual arousal or attraction.
2. If they're gonna cheat, do it now
I would rather that if my partner was to cheat, that he would do it now, rather than when we are married with kids. It's best to expose them to temptation and see how they react.
3. Can't keep a wandering eye, from straying
A wandering eye will wander regardless of whether you keep attractive friends away. If he/she wants to cheat they will find a way, so may as well allow them the freedom and opportunity to show you their true colors.