I reuse code a lot. If I write code in Ruby, I can't reuse it in a Python project. Since Python and Ruby occupy the same niche, I will often want to reuse code.

(This is less of an issue if I want to use Python and C++. I'll rarely want to reuse linear programming code or a PDE solver in Python, and if I did, I would just wrap the C++. Similarly, I never want to use code I've written for the web in C++.)