I agree that having a happy, fulfilled wife would be a major benefit. Also, not having to watch her suffer as she tries to suppress part of herself to make you happy.
My husband tells me that when I have an OSO, I am happier and more relaxed in general, so that benefits him just by having a better me around. I also tend to be more appreciative of the things that hubby does since we don't get as much time together (love languages thing - when our quality time decreases our words of affirmation or acts of service or gifts increase to make up for it). That change up in how we show our love and affection is nice every once in a while since it keeps us from getting too complacent and habitual in our efforts. When we are spreading the love, we are more proactive about making sure our feelings are clear whereas when it's just the two of us, we tend to get into a routine. Not that we don't feel loved during the routine times, it just isn't as flamboyant.