I actually think that its very naive to think this 2nd analog add-on will be a must-have requirement for 99.9% of the games that are made for the 3DS either now or in the future.
It's clear to me now, that the slide-pad analog is merely an option. An option designed exclusively for users that can't handle 1-analog gaming. The ones that grew up with the dualshock. For them, Ninty made this add-on.
And its not comparable at all to the Motion Plus add-on which was required to play Wii games designed to work with it. For instance, you will not be able to play the next Zelda without the wii motion plus. Yet you will be able to play every single 3DS game now in development even if you don't have the 2nd analog add-on. So there's no comparison.