rebase --merge question

Hi all,

 From help rebase:

--merge
	Use merging strategies to rebase. When the recursive (default) merge 
strategy is used, this allows rebase to be aware of renames on the 
upstream side.

Renames of what? Files I assume. Are there any disadvantages compared to the normal rebase? If not, why isn't --merge the default behaviour.

Thanks,

Stijn