Ref T13272. Currently, the hard-coded default homepage looks like a dashboard but is actually rendered completely manually.
This means that various panel rendering improvements we'd like to make (including better "Show More" behavior and better handling of overheated queries) won't work on the home page naturally: we'd have to make these changes twice, once for dashboards and once for the home page.
Instead, build the home page out of real panels. This turns out to be significantly simpler (I think the backend part of panels/dashboards is mostly on solid footing, the frontend just needs some work).