Am I on the right track:
Instead of just using the XDA 'components' to add to the toolbox one at a time,
Is the reason I should use User extensions is because I can likely package
up multiple components and sass and resources (and themes?) into one XUA file rather than
have to load multiple items from separate XDA and resource JS or CSS files?
Otherwise making a user-extension with the Def files seems like a lot of detailed work.