My first thought is concerning potentially writing posts offline. I think I would like to be able to query my micropub endpoint and get an array of contacts, in the same way I can get an array of syndication targets. This allows the ability to be writing a post offline and to power either person tagging or @-mentioning.
Right now you can only have one draft post at a time (if you click cancel while writing a post, it will ask if you want to save you post as a draft, if so it will auto-fill the form when you open it back up again).
Eventually I would like if you are offline to click "Post" and to have it auto-post when it gets internet again. So in those cases I would want to be able to handle all nickname cache stuff on device after the initial login (and maybe periodically refetching the cache every couple of days or something).
I can see how it could be useful to query the server with the string so far in a situation where you know you have internet.