As you say, there are many situations where wielding two weapons outweigh the benefits of a shield. (As an aside though, there are shields that add a lot of dam too.)

However, there are also situations when having a shield is better than having two weapons. Going to fight a shaman? who uses preps? Give me a shield that I'm not going to lose to -str any day, and something like a single light whip, rather than a pair of nasty axes that I'll find myself unable to wield, be blind and therefore stuck weaponless.

Dual wield is generally better than using a shield. That's one advantage given to classes with dual wield. Classes without dual wield have other advantages to make up for it. Like immolation + anti-lag shields, or sleep + -str (rendering the dual wield history), etc.