It really does make no difference because the numbers are truely random and are used only once for each game- you can't bet against another player, only against the house, so it wouldn't catch on.
Your argument is similar to saying that we don't need the 'red' bet on a roulette table, since there's no difference between 'red' and 'black'.
Mathematically that's true, but players like to feel like they have some control. If you feel the lucky number is going to be high next time you play, wouldn't you like to be able to play "over 32767" instead of "lessthan 32768"?
And how many people would bet each way (both over and less than x) and think they have a fool proof way of beating the house? It might take over from martingale strategies.
Looks like someone has learned how to cheat satoshiDICE - 
