I used to write a lot of python, and the hard part is that as you write code you're pulling in more and more external libraries, and sooner or later one of them will be incompatible with python 3. Then if you want to continue using python 3 in your own code, you need to spend the effort to port that library to python 3.

You don't invest time to learn it, you invest time by writing or converting code.

Writing code for Python 3 and 2 isn't that difficult.

I've been doing it for a while now while working on a variety of different libraries for the Pyramid project, as well as hacking on WebOb which is Python 2 and 3.

Especially if you drop Python 3.2 and lower from the Py 3.x series you have very little trouble writing code that runs on both (with a shim or two here and there).

It's not nearly as difficult as it once was.