Ubisoft requires an "always on" connection; the game pings a server constantly and pauses/quits if there is no response. Having a download running in the background, for example, is likely to disrupt your game, and some games, like Splinter Cell Conviction, don't even need that to be flakey.
Steam just requires you to login, after that you can use it, even if the connection fails. You can even restart Steam in offline mode, so you don't need a connection at all anymore. You need to login online before you can switch though, so the argument some use that Steam games don't require a connection at all is simply false, at least to some extent.
* darnit Jim
