I would literally laugh at anyone that used some passive aggressive device like that. If you want to use a clipboard or your laptop to deny yourself the armrest, that's cool, but if you're going to actually attach a purpose-built device to "solve" a "problem" that can 99% of the time be resolved with a simple request to your seatmate, then you deserve to be made fun of.
I agree that it's kind of ridiculous to buy something for 40 bucks that a 15 buck Moleskine can do just as well, plus you can write or draw in it.
But still I'd rather hear you laughing than get your chicken wing poking me in my ribs.