As someone that is married that also has my bestie living with us I TOTALLY see the benefits!! My husband doesn't have a sexual relation with my bestie but we are very much a family of 4. 3 adults 1 child. We equally parent, have two incomes, I stay at home. (bestie contributes to rent and bills only, no child expenses unless she buys him toys etc by choice)
It's fantastic! My husband works long hours. I have company, she loves living with us, we share chores, she helps with ds. It's the BEST. We started calling her 'mum J...' as a joke because people think we're genuine polygamists. It has stuck and she really is 'mum j...' now haha.
I think I could honestly come around to being a proper polygamist!