The way I'd look at it... anything from the RM you download or purchase definitely do a child style. If you'll get any updated XML from an author you'll do definitely want to do a child style.
Merge feature won't work when overwriting an XML like that it doesn't give you an option to "merge" your changes to the updated XML. With a child style of the "recently overwritten style" the merge feature would work because your merging your changes into the parent changes.
Hope that made sense