I do not see where is a problem with porting if the files are with mixed-case naming?
There is no problem when a mixed case unit name matches exactly with the mixed case filename everywhere.
Problems come when unit names are spelled a little differently somewhere. Pascal language is case insensitive. Lowercasing filenames is a strategy to allow different Pascal spellings.
I have seen the problem many times in example projects sent by windows users. I must fix char case before it runs on Linux.
The new menu item is especially useful with Delphi projects ported to Lazarus. It can save lots of manual editing. Yes, the Delphi converter should have an option for it, too.
Apart from renaming the files back to the mixed-case (after Lazarus renames everything to lowercase), one also need to edit the .lpi file to match the file names, because there is also everything lowercase.
The settings mentioned by WooBean seem to work fine. Just turn off lowercasing and you are good to go.
If you find a bug there, please report.