The most compelling reason would be to put something on that would make me want to see it clearer. I'm perfectly happy with my analog tv, although occasionally I would like a big-screen for certain movies.
Want a better picture? Switch to the DISH, I did last Saturday, horizontal lines of resolution from 400 to 525 lines, near DVD quality, standard broadcast from your local stations anout 280 horizontal lines of resolution.
Our existing cable system is good, but the DISH picture quality is much better, plus 169 stations (including the music only).
(Freepmail me if you want more info.)