If there's changes, you want the most up to date version. The vast majority of the map doesn't change in any given interval, but you don't know what areas did change, unless you want to send down a changelog stream.

That said, if you don't like what Google Maps does, there's lots of other choices. Here maps (from the Navteq/Nokia lineage) had pretty reasonable downloading options when I was using it on WP and is available on platforms that lived.

What is the utility of a map of all of Great Britain if you don’t also have the detailed data at a city and road level? It would be a lot of data if the apps were to try to cache it all the way down to the lowest zoom level. Should the apps always cache everything? If not what would be the cutoff? If you don’t have all the data, what benefit is the outline of the island?