The real problem with "don't break the chain" is that once it does, things collapse.
See my graph: https://github.com/steveklabnik
As you can see, I'm about to lose a ton of green. I'm at 87 days as my longest, but July 6, 2013 was brutal for me. I was actually flying, and had saved a small bit of work to do during a layover, but then I totally forgot.
Once that chain was broken, it was super easy to justify taking some time off...