You should always ask yourself, why the code you are going to write should not be open source? It's only true for paid projects and may startup prototypes. Paid projects should be not problem because they are paid. That leaves the startup ideas.
I'll take a stab at this since I signed up for bitbucket after reading the comments:
I have a lot of code. A LOT. It's mostly random things around data to make tracking things in my life easier (diet being the most recent one). And most of the code is crap in two ways: (1) I'd never want to be associated with it in a public sense and (2) It's so specialized to my life that it's unlikely that anyone would use it or contribute to it. Sure, there might be a few hundred people in the world that could find it useful, but those few hundred are unlikely to happen upon my kludged-up solution, and if they are, I'd feel badly when it doesn't work for them the way it works for me due to my personal configuration being hard coded throughout.
