There are a few scenarios on the client where knowing a title's namespace is important. For example, Android has special rules for File: and Special: pages, and the iOS app has issues rendering titles outside the main namespace (T119010) and wants to exclude them from it's "Explore" feed (T123577).
Returning this information as part of the mobileview action is a backwards-compatible way to give us this information. action=query already returns a ns field which contains an enum for the title's namespace. Adding a similar field to the mobileview response would satisfy the needs listed above and allow for removal of hard-coded prefixes for multiple languages.