I don't know about common, but for a couple that knows they're for each other and doesn't have jealousy issues it's not a problem. If you've ever had a conversation with an SO about a "celebrity pass" or even about sleeping with someone you know who would it be, it's like that only taking it to that level instead of just talking about it.

For my ex and I, we mostly met well educated couples that were successful and discrete but had been together since high school and were bored. We had some very good friendships with a handful of couples.

In the five years we were together I never once questioned her fidelity, and if she ever questioned mine she never did it openly. It's a nice thing to have in a relationship.