Hacker News Reader

The biggest reason is in case the original source goes away (the author removes a particular version for example). A lesser reason is to make it easier for others to get up and running without needing the build tools.

I tend to leave them out if it's a personal, open-source project. For a commercial project I tend to check them in.

I guess if that were the case I'd want to package the dependency with my code.

I feel like I'm either providing my app with a dependency framework for people to build

or

I'm providing a packaged product to the public.

I suppose you can use the same public repo for both purposes but I would prefer to separate the two functionally.

If you're doing something more than a pet/toy project then there are plenty of reasons why you would want to check in components.

You still get easy control of upgrades, new installs, etc. The tool is there when you want it, instead of on every new clone.