arguments to freeze are used to filter  installed packages

Any arguments to the freeze command used to be discarded. That makes freeze less useful in finding which versions of a few specific packages you have installed, especially if the list is long.

This change makes pip freeze only output that have an argument as part of the package name so you can do:
     pip freeze xl pymongo
and get 
    openpyxl==1.6.1
    pymongo==2.5
    xlrd==0.9.0
    xlutils==1.5.2
    xlwt==0.7.4
and not a list of 158 packages. The comparison is done lowercase.

You can of course pipe the output of pip freeze through fgrep , but especially when trying to select multiple patterns in one go, that is not trivial.