This is a really great idea. The doc contains some nice layout of use-cases for mobile users sharing and loading landmarks. I should be able to easily send someone a location, especially my location, our meeting spot, a great hike, or whatever. However, it's not clear why Nokia defined their own specification. Or at least, why would they continue to push their own specification.
One of the purposes is interoperability - as stated in the specification:
Landmark messaging should work across different manufacturers' devices. This specification is open and license-free and allows incorporating the functionality to different software platforms and interoperable Java MIDP landmark messaging applications for the devices of other manufacturers.
While the specification is "open and license-free" it is still controlled by Nokia. Look at the MIME-type:
application/vnd.nokia.landmarkcollection+xml. What that means is that while you are free to use the spec, you probably won't have any say in the format if you need some functionality or find a problem.
Now compare to a location-documentation format like KML. Now, to be fair, KML wasn't as ubiquitous in 2005 (or the year Nokia was probably working on their spec), and the MIME-type of KML is still
application/vnd.google-earth.kml+xml. However, KML is now undergoing external standards adoption, and will be outside the specific control of a single entity, Google.
More than just ownership
While allowing community input into a format is nice, there are more reasons to use a more widely used format like KML - namely that it's more widely used and therefore there are a lot of clients and services that can make use of the format. More players means more fun.
Nokia's Sports Tracker app does export KML, as well as GPX. This makes it very nice to email, share, and upload your hikes or workouts. Perhaps this is a trial for them. However you can't load KML into Sports Tracker to follow someone else's track.
Here's to hoping that Nokia supports KML and even GeoRSS for subscribing to a source of location information (such as a stream of sensor data or friends' locations).