1) One subscription, multiple devices connected (yes you can get that with connection sharing e.g. PDA.net, but...)
2) You can talk on your cell phone and not affect your data connection (or vice-verse)
3) You can lend it to a family member or friend and still hang onto your phone.

When I got my Mi-Fi it was free with a 2-year data plan, which can be suspended when I don't think I'll need it for a while... Don't know what the current deals are.