I, too, was wondering about the function of the fourth direction, with only three taunts. What I figured was that there are two possibilities: 1) it's right on the D-pad, which would mean it's awkward to reach, so he left it out intentionally, or 2) it could have miscellaneous purposes, depending on the mode.
Before I saw in video how assist trophies worked, I was thinking that you could choose an assistant before battle, and picking up a trophy would let you call your assistant with the D-pad. Then I was thinking that it might be possible for Brawl to include a tag-team mode, which lets you switch your character in real-time during a match; this could be done either by having a human-controlled partner, or by letting you actually switch the character you're playing. I think that would've been an interesting mode.
But I don't think there's really any way Sakurai would give the fourth direction a useful function. How would you even use it with the Wiimote?