Yeah, Smith can anchor an offense where he throws under 30 times a game and takes sacks instead of deep shots.
His best attribute is that he knows how not to get in the way if the team is loaded with talent. You can go with that type of quarterback in a rebuilding period, but it makes no sense to give up significant assets to get a placeholder. This is Kevin Kolb and Andrew Cassell all over again. In fact, I'd say that this is worse because at least those guys had the illusion of untapped potential. This is more like the Carson Palmer trade. Old starter who is no longer in the plans gets dealt for a highway robbery return. At least Carson Palmer used to be good once.
Harbaugh knew what he was doing. He drafted a replacement as soon as he could and jettisoned Smith the first chance that he got.
The fact that you call Smith a placeholder shows how little you know about the relative value of a QB. At worst, Smith is an average QB, but that doesn't really do it justice as to how incredibly difficult it is to find even an average QB in the league. A few select QB's become Tom Brady or Peyton Manning. More of them become Alex Smith or Andy Dalton, and the vast majority of them become Blaine Gabbert, Christian Ponder, or even worse, JaMarcus Russell or David Carr.
If you can get your hands on an Alex Smith, you mind as well do it. If the Ravens just won with Joe Flacco, any team can definitely win with Alex Smith as its QB, as long as it has the right talent around him. But then again, that's what you can say about every QB in the league, including Brady or Manning.
Also, if you've watched Carson Palmer play at all the this season, you would know that he's still every bit the QB that he was when he was in Cincy. The guy can play.