One of the questions I've been working through in my book is this: how do you decide when it's okay to outsource a cognitive function? When is it okay to let your electronic address book remember all your phone numbers, for example? When should you try to memorize a street address, rather than let your GPS or iPhone remember it for you.
I think the simple answer is this. Will memorizing the information help you survive a Zombie Apocalypse?
Let me explain through a couple examples.
I haven't memorized a Skype user name other than my own. Ever. And I don't worry about that one bit. Skype user names are useless outside of the Skype service. You can't go on AOL Instant Messenger and call someone's Skype username; you can't dial it from a phone; you can't use it to send an email. The only context in which that piece of information is really useful is when you're using Skype. (You might argue that if I know someone's address, and a different person asks for it, I could give it to them; but poor mental cripple that I am, I can't. My response is that Skype itself contains pretty powerful search functions that let you find pretty much any user who wishes to be found.)
Now, if there is a Zombie Apocalypse (or ZA), I assume that Skype is going to go down. The service might keep going for a little while, but eventually its sysadmins and developers are going to stop maintaining the system, and start eating brains.
Knowledge about usernames holds no value outside the context of the service. Given this, it makes sense to not bother to remember usernames.
More generally, knowledge that is useful in one specific context only can be safely left out of your memory, if it's easily retrievable in that context. For example: pretty much every weekend I make pancakes or waffles. I always use Bisquik to make them. The recipes for pancakes and waffles are on the side of the Bisquik box. So even though I've made these for ages, I still don't really remember the recipes. Why? Because if I don't have a box of Bisquik, knowledge about how to cook with Bisquik isn't really very useful. And I can rely on the Bisquik people to remind me of the recipes, by publishing them on the side of the box.
So like Skype, whose usernames are useless if the service doesn't operate, I don't memorize the Bisquik pancake or waffle recipes because they're on the side of the box. If I don't have the box I don't have the mix. and if I don't have the mix, I don't need the recipe. The information and its utility always reside together in the same system.
In contrast, I feel uncomfortable if I don't know the phone numbers of close friends and family. (I confess I haven't memorized them all, but still I think I ought to.) Likewise, there are lines of poetry or quotes from the Bible, Stoic philosophers, and elsewhere that I memorize because knowing them makes me feel like a deeper person, and because they're useful during challenging times (like during a ZA). And while I take thousands of pictures a month at sports events, school functions, on trips, and so on, I still think it's very important to remember those events– to construct an interior narrative that gives them some logic, and places them in a structure that helps explain them– not just have records of them. As some neuroscientist said, our experience is reality is a vanishingly thin edge of the present, behind which stands a vast store of memory. Looked at this way, memory isn't just a function, or information. Memory is you.
Knowledge of how to fashion a tent: useful. Knowledge of how to assemble THIS particular exotic tent? Probably less so. Need to learn it if the instructions are stamped on the side of the tent? Zero. The need to be able to remember events in your own life, so you can make sense of yourself and the world. Always infinite.