Thanks for that. I didn't really expand on that point in the article, but yeah, I can see quite a few uses where an app creator would want to update the Live Tile when they don't have network access.
Take as an example a Tamagochi-style app. If I were writing it, I'd like the Live Tile to update with the current state of my Tamagochi creature (Is it hungry? Lonely? Playing? Sleeping?), all of which the app on the phone knows. If Live Tiles could be updated on a schedule via a callback into my app, it could generate the Live Tile on its own (Silverlight is great at graphics, after all), all without ever needing to connect to the network.
However, that's not possible with the current Live Tile mechanism. Right now to create an app like that I'd have to maintain a dedicated server infrastucture to keep the data up to date, I'd need to duplicate the game code on the server (so it would know what to do as time elapses without the game being launched) as well as needing the image-generation code on the server to post the updated Live Tile image. That all seems horribly unnecessary for such a simple application.
I could maybe kludge around some of this by having the app send itself a notification of a new Live Tile, and have a handler for the Live Tile URL on the client generate the image on the fly and return that when the phone asks for the Live Tile. I haven't tried that, but it might be possible. Even if it is, it seems horribly contorted and the network serves only to introduce a (fairly flaky) point of failure.
That said, there are plenty of drawbacks with generating a Live Tile via a callback into the code. How would it be handled? A callback that just took a closure would be really nice but very difficult for MS to implement, so maybe a self-contained interface/type that had access to the same isolated storage would be better. Even with that in place, MS would need to put strict limits on resource usage, just as it does with the time allowed to fetch a Live Tile and the size of the Live Tile image file. Maybe it could give the thread a five-second limit before aborting it if it hadn't generated an image?
I have no intention of ever creating a Tamagochi-style game, but I thought it was a nice example of one case where I'd rather be able to generate the image locally and never need access to the network.