In the comments thread for Q&A: where can I buy Office:Mac 2011?, a commenter named Mark discussed the differences between identities in Entourage and Outlook, and said that the changes would mean that he won’t upgrade to Outlook.  Here’s part of his comment:

I, as well as others I know, share a computer with a partner. The main reason we use Entourage instead of Mail, was the ability to easily switch identities, so that each of us can separately check and view the various personal and business accounts we have.

In Entourage, the identity is where all of your data is stored: accounts, preferences, your data, everything.  The identity is an implementation detail that most applications wouldn’t expose, since most people have little reason to care about how this kind of thing is stored.  However, back when Entourage was first introduced, there was another purpose for exposing the concept of the identity: multiple people who share the same computer.

Longtime Apple users will know that the ability to quickly switch between accounts wasn’t originally part of OS X.  So in the case of email, if multiple people wanted to share a computer but keep their mail separate, it was pretty difficult.  Entourage’s identities allowed for this to happen.

But the introduction of fast user switching in Panther changed things.  Instead of having a single user account, it’s very easy now to have multiple user accounts for different purposes.  Each of those different user accounts can then run Entourage or Outlook to get their mail.  Switching between users is quick and easy.

As we worked on Outlook, we made the decision to continue to support multiple identities, but not to make it quite so obvious.  We don’t need to expose this implementation detail to the world.

Personally, I also have a shared Mac at home, for me and my husband.  For our shared Mac, we have a shared account (named Tipsy) which has all of our shared stuff on it: Netflix, Quicken for our joint finances, iTunes (and then we have playlists for syncing with our individual iPhones and iPods), iPhoto, etc.  Then we each have our own personal accounts for storing our own information.  For us, this is more about ease of access than privacy; he has his own organisational scheme and I have mine, and so we don’t force each other to figure out the other’s scheme.  If I need to access my stuff on our shared computer, I just quickly switch into my user account to get it, and then I usually return it to our shared account when I’m done.  We use different desktop backgrounds for the individual accounts so its easy to see which account is currently the one that has focus.

For my work computer, I also maintain different user accounts.  I have my primary user account, which is where I spend most of my time.  I’ve also got a demo user account, which I have populated with a bunch of demo documents and a couple of test Exchange accounts.  I often use my demo account when I’m giving presentations too, so that I don’t have to worry about whether I’ve logged out of Communicator.  If I’ve got Outlook running in both of my user accounts, it feels faster to switch between my user accounts rather than the old Entourage model of switching identities.