I think adding sources would not only be polite, but make it easier for interested readers to delve more deeply into the matter. E.g., as in https://gist.github.com/jboner/2841832
Oops, I made a similar comment before seeing yours. Glad to find someone else peeved enough about it to point it out. It's such a small thing but I don't understand why it has become more or less acceptable here to just blurt something out in a long blog post without any references whatsoever. It might not be malicious necessarily however the standard for technical writing needs to improve.