That's interesting. Personally, if I knew something like that, I think I'd make sure to highlight points where I'd compare favourably to the other candidate, but I'd leave it to the interviewers to actually make the comparison, rather than telling them his weaknesses.
Now, that's more civil than the world of advertising, but it's the same general principle. Commercial tend to make it a little bit more humorous. Comparisons aren't always polite, but they're often appropriate.