Since we are talking about blocking, it does seem as if they sometimes do it - or maybe it's just the luck of the draw (I am only GO).

I say this because of a recent experience - I had an aisle exit seat (I think it was an A320 or A321) and both middle and window seat were empty. Towards the end of boarding, the FA asked me if I minded if someone moved to sit in the window seat. I said it was no problem, but I was amazed they would ask me.