I was on a crowded cross-country flight with a co-worker who was unable to sit with the armrest down.
She was embarressed and we were both uncomfortable.
But you have to admit, the seats are very small, and our airlines have the oldest planes in the world. The seats are not pleasant if you're tall either.
I would hope that if you have to purchase two seats, you'd get airline miles for both, which would provide first class perks much sooner.