On one hand numeric UIDs are bad because they imply someone with lower UID is better because he's been around longer (which is not necessarily true). On the other hand seeing a handle with a brand new UID tells the user is new to the site or it is some fake account. This helps in story voting. I vote "Dump it" for all stories submitted by people who have no diary and only a few comments (and usually the comments are on his own story).
In short, I'd say hide the numeric UIDs. I usually check if people have comments or diaries so seeing their UID isn't important. Even less so because it is not immediately visible on the story submission or user information pages.
Since the handle is fetched for comments and stuff anyways, there's no point in supporting URIs which contain numeric UIDs instead of the handles. For the DB it is just the same if the query is performed with handle or UID.
