It's a chaotic world!
While the SteriPEN is a rugged and reliable device, we recommend bringing some kind of backup. Think of it like being on a yacht. It's not likely that the yacht will sink, but it's a good idea to have a life jacket handy, just in case. On the same note, it's not likely you will experience an issue with your water purifier, but it's a good idea to have backup.
If you purchase a SteriPEN or other water purifier, purchase another purifier of a different type; having water is that important.
For example: If you prefer to use a SteriPEN, bring a bottle of iodine as a backup; if you prefer a filter/pump, bring some chlorine tablets; just make sure there is no single point of failure that can prevent you from having purified water.
Have an extra set of batteries
Believe it or not batteries can be faulty, and it just takes one out of the set -- 2x CR123, 4x AA-- to prevent proper operation of your SteriPEN or other device. Also, you don't want to be in the middle or nowhere and discover you've used all the available doses from your only set of batteries.