Thanks for the comments. Whatever Ushiro's intent, the heel "stomp" is one of those actions that gets misinterpreted. It is part of an older system of combat that precedes aikido, and therein lies the problem: the loss of meaning as an action is transmitted from one "generation" to the next.
It reminds me of the old story of the housewife who always cut the ends off a roast before putting it in the roasting pan. It looked kind of funny, a roast sitting in a big pan with its ends lying separately on either side. When her daughter asks her why she did that, the housewife answers that it's because that's what her own mother always did. So the girl asks her grandmother about the "tradition" and the grandmother replies that her old roasting pan was too small, so she had to cut the ends off to make the roast fit!