Aftertouch is primarily used to introduce vibrato on a sound, or occasionally open up a filter on pads, but it would have little effect at all on a quick run. You usually have to hold a note at least for a while to be able to add the extra pressure (it's nothing to do with speed) needed to make the aftertouch effect noticeable...

On another note, I'm also not a big fan of aftertouch triggerable OS actions, like fills or variation changes. Maybe it's just me, but I play a bit too heavy to avoid accidentally triggering the action when I DON'T want it. After all, the aftertouch value is the SUM of the pressure on the keys all over the keyboard (virtually no keyboards out there that can distinguish between pressure at the top or the bottom end of the keyboard). Some others seem to get on it OK, but I'd be surprised if they came from a good piano playing background. Organ touch is light enough to get by I suppose, but piano requires a bit more 'force', not just speed, IMO...