Sharing your location is, at least right now, more of a feature than a product in users minds. If you have a google maps client on your phone already, you'll probably try it. Users probably won't download an app that does just that as readily.

Google has the classic big company/large install base edge in this area.

If sharing the location is not the neat feature - perhaps it's visualizing/mining that information accrued over your daily life/time. Here's some concepts from MIT

http://web.media.mit.edu/~orkan/projects/portraits/main.html

Completely agree. It's a feature, and if it was a free add-on, it'd be useful.

eg if someone is going on a long journey, be able to track them to see they get there safely etc

But I don't think it's anything more than a feature.

Yahoo has also tried too, not to much success though: http://fireeagle.yahoo.net/

Except fireeagle doesn't have a client with an install base as large as Google Maps' and is really more of a pipe/aggregator than a client in the first place.