It may cause errors when using twill as a library with other libraries.
For example i had problems with pyparsing, need pyparsing 1.5.5, and i have it installed but twill updates sys.path to use it's own version of pyparsing 1.4.8 and it causes an error for me.
The Jonathan Rogers updates (https://github.com/ctb/twill/pull/3) repair the same problem with a solution I would find better: get rid of other_packages and use updated packages.
It may cause errors when using twill as a library with other libraries.
For example i had problems with pyparsing, need pyparsing 1.5.5, and i have it installed but twill updates sys.path to use it's own version of pyparsing 1.4.8 and it causes an error for me.