Hi!
Yeah, there are some weird (but apparently popular) suggestions there.
It's an interesting question whether we should or can do anything about it. On one hand, the potential for hilarity is obvious. On the other hand, if that's what people look for... On the third hand, obviously different people look for different things, so here one-size-fits-all may lead to weird results. Also, visits and searches are not exactly the same... Maybe we can get search stats or search click stats or referer stats or something like that instead? Another option proposed would be to assign negative category boosts, but then we would have to manually curate the "bad" categories.
One other oddity: Searching for a digit works great for 1 through 9. But searching for 0 doesn't bring up ANY results, either for the suggester, or for prefix search. Doing an actual search on enwiki for 0 brings up a disambig page with lots of reasonable candidate results.
That may be a bug. I wonder if we don't have if ($search) somewhere that leads to it - since "0" is falsy in PHP, it may make it look like "". $a != "" would work though.