Seems most useful for information that changes often. Say, for example, you move around a lot and I want to know when you're in town. I could poll your API (and others like you) and see who's in town this week.
Or maybe, as a freelancer it could be used to signal that you have free time. And the consumer could then find 5 rails programmers, who also know whatever, with at least a week of free time coming up.
Sort of responding to you and Nick Lothian (nl) at the same time, but that's the main difference I see between something like this and webfinger, is the ease of updating.
Further, I see a distinction between Personable (the site) and the "Personal API", is that I see Personable as also having services that actively monitor other social sites and update Personable automatically. Checkins to Foursquare, Facebook, etc could be automatically read in to Personable, which would show your 'current location' as whichever the most current update is, regardless of which provider you used.
Same thing with RSS feeds, Youtube videos, etc., etc.
Perhaps not much of a distinction, but that's my thought process anyway.
Or maybe, as a freelancer it could be used to signal that you have free time. And the consumer could then find 5 rails programmers, who also know whatever, with at least a week of free time coming up.