When asked to create system XYZ and you ask to do it in Python over PHP or Ruby, what are the main features you can mention when they require you to explain it?
I agree with mreggen. Tell them by working in Python you can get things done faster. Getting things done faster possibly means money saved by the client. In the least it means that you are working with a language you a more comfortable in, meaning faster development, debugging, and refactoring time. There will be less time spent looking up documentation on what function to use to find the length of a string, etc.
How to sell Python to a client/boss/person