I sort of copped that one on a recent VA short-haul international. The +1 and I had specifically chosen row 4 and it was both a very quiet flight and I knew I'd get a shadow anyway, so B was spare. But in fact all of row 3, A through to F, was completely empty. The fixed armrests made it a deal killer for the two of us lovebirds, but lo and behold here come some enterprising pax up from the rear, who grab some of row three and then recline all the spare seats into 'our' space!
Bad form. Not necessary.
Another scenario is DEFG seats, if DG only occupied, extremely bad form if EF were reclined. In fact recently had that scenario (EF empty in several rows), travelling SIN-SYD, as the person in front of me reclined immediately after the wheels were off the ground, and shortly after (ie seat belt signs off) I moved to E. If he'd reclined E I would have definitely asked him to put the seat up, as I did have my laptop out for part of the flight. He didn't thankfully.