Um... you don't have to switch to right thumb, but it makes for superior key balance, and its not really as hard a switch as adjusting any other finger.. because you're probably not already using your right thumb on any other key.
For example, if suddenly I want to start using my ring finger for " i ", that would be a large switch because I'd be making a large shift in many words and rhythm.. to use it in a different way also means , you're actively remembering to cancel a predefined muscle reflex which will take time to disassociate.
Whereas the right thumb doesn't do anything, so it's unencumbered and free to adapt easily.