I'm always stunned when I find myself defending the airlines, but... as you probably know, they generally charge more for tickets the closer you get to the travel date. On the day of travel, the "walk-up" fares may be multiples of what you paid. Naturally, they'd rather charge someone else the walk-up fare for that seat, or give it to a standby who missed a connection, than sell it at the same discounted price you paid for the other seats. It would also mean juggling the seat assignments around. Let's say you want 3 across plus the aisle on the other side. The person in the aisle seat on the other side will have to be moved. He/she undoubtedly reserved it months ahead and will not be happy at being moved to a middle, which may be all that's left. I've read enough sad tales on this site to know that even when families reserve 4 or 5 seats close together well ahead of time, they can get to the airport and find that the 3-year old has a seat 4 rows ahead, on the other side of the plane.

Not a bad idea, though, if you execute it ahead of time and keep checking to see that they haven't trashed your seat assignments. My husband is also over 6 feet tall and he turns 72 next week- we may try reserving 3 across sometime if we don't want to spring for Business Class.