Well if you can get a spare seat next to you, why not?

I had an experience on an overnight Y class flight on another airline where the airline had blocked the seat next to me. It turned out to be virtually the only spare seat on the plane. The FA came and said he would have to put someone there because the couple had a baby and there was no room for the baby. I didn't argue but I was pretty peeved - when the guy came and took the spare seat, I realised he had left a screaming crying baby with his wife in the front row of the cabin. Why should I lose a seat because he doesn't want to sit with his wife and screaming baby?

The FA came later and apologised after he realised I was a top-tier passenger on that airline.