On flights that we know have 10 or so empty seats, spouse and I will take aisle and window of exit row. We never put any personal stuff on that middle seat. Every time, the FA stands in front of that seat during boarding, effectively holding it open, unintentionally, for us.
I don't know why people don't just ask the FA if they can sit there.
On a side note, last week, out of PHX, the FA had put down all the window shades in the bulkhead row, to minimize the hot sun coming in.
No one sat in that entire row, throughout A boarding, and half of B group, and no one even asked the FA up front if the row was held or what?!