Yet, apparently, there is some emerging science to suggest it has a purpose in design. Well, at least according to one Gawker columnist:
Sometimes, there will be ugly fonts, and that's okay. And finally, we have the science to prove it. A new study indicates that difficult-to-read fonts can help people learn and retain information, possibly because they require more attention and work to process and understand.
The article then quotes a few studies where two selects were given a document in Comic Sans, and the other a normal font. In those tests, the Comic Sans select showed greater retention of the material.
While I'm not doubting the results of the tests, nor the conclusions drawn from them, I think they miss the point. In advertising, reading/viewing the ad is largely voluntary. This is especially true in direct marketing.
Comic Sans may increase retention because of its difficulty to read; however, that would only be useful in the case where the audience was required to read it. Nobody will voluntarily read an illegible font. You don't have that amount of time to sell your idea.
So, for use in advertisements, it's still a really, really bad idea--regardless of how stupid you would look as a designer who used it.
As a general rule of thumb, never use illegible fonts for a logo or in design.
(Link h/t: @Odney)