The first reason to engage in such tactics is that the Bushies wont let themselves be bound by delicate notions of honesty or ethics. In the more than 20 years I have been observing politics, the Republicans have only grown more shameless, while Democrats have occasionally experimented with "taking the high road." Ask Michael Dukakis and Paul Tsongas how that worked out. The American electorate has had plenty of opportunities to reward politicians who abjure pandering, negative ads, and propaganda, and it has consistently failed to do so. Instead of employing campaign tactics for an electorate I would like to have, I would rather target the electorate we actually have.
The other reason to circulate draft rumors is that, while objectively false, they evoke emotional responses that correspond to a deeper truth. The Bush Administration has repeatedly denied a need for the draft, but the most plausible scenario for a return of the draft would almost certainly be an ad hoc decision forced on the Administration well past the point of prudence. Raising the issue of the draft focuses public attention on this failing of the Bush Administration in a way that is as accurate as it is effective: the government of George W. Bush is typified by incompetence and zealotry, and when its policies fail, rather than acknowledge reality its first response is to try to obscure its error by squandering American prestige, treasure, and blood.