I think Philip's reasoning for doing this (though it's been a while since I read it) was to make future upgrades easier, by not changing the existing tables. However, upgrades have always been such a PITA anyway that I have always just added to the users table (and documented my changes for posterity). I didn't feel the mythical upgrade benefits were worth having to have an extra join every time I used that new information.
It's possible that in the future upgrades won't be as painful as they have been in the past, and this will become bad advice, but I think upgrading software like this is always pretty hard. What we have done so far is start with a clean install of the new version and migrate over all our changes; if you're careful to document them well then it's not all that hard to do.
