I use Prefetch. It is a good feature. I did not realize how deep it worked until just reading the WIKI. So I now prefetch from multiple levels. Since my laptop in not online normal, I prefetch whole areas at a time.
Here are some thoughts.
Is there a reason to limit the prefetch depth? Why not allow the user the ability to download to any depth?
In other software I have, the user selects the zoom levels for tile retrieval.
In that same package, the author has also limited the zoom levels to:
1/4M, 1M, 4M, 16M, 64M and 256M per pixel. The maps are USGS topo, aerial and urban tiles from the TerraServer.
I was also thinking the ability to prefetch all tiles below the current level when the map is moved is possibly useful. Obviously this can be very time and space consuming. But if the user could select the levels he/she needed, then only the levels select would be fetched.
One reason this is useful to me is that I run a GPS simulator and have been retrieving tiles automatically. This has been useful so far in getting tiles for areas that I am interested in (mainly traveling to and from and though) and it would allow a single pass.