I am more like your nephews parents in that I don't have my kids watched. Not because I'm against anyone, I just don't have that need to be away from them. I would rather spend the night with them, than have them watched and go out. So if I left them with relatives, it's more for the relatives than me. I'd probably end up just sitting and counting the hours til I could I pick them up (I'm always super early for school pickup). So it's just not something I would do. And I would hate to think my husbands siblings (I don't have any) would think it's something personal.
On the other hand, my SIL and her partner are about 8 weeks away from moving directly across the road from us. I already anticipate babysitting her lot all the time as she loves going out for date nights etc. and already had family that lives near her babysit all the time. And I do love the idea of that. So I do see your side too.