The full strength team will include Symonds and Clarke, as well as Watson, so there's certainly backup bowling there.
The issue with Tait is simply that if he bowls as well as he can bowl, which most Australians will have seen in domestic cricket and perhaps once in an ODI, Australia's attack suddenly looks extremely good, even without Lee. Throw in what is (IMO) fairly comfortably the strongest ODI batting lineup in the world at the moment and you've got a team that should really win the WC. You can't say that about anyone, as has been implied, because few bowlers have the ability to turn a game which Tait has demonstrated from time to time in domestic cricket.
If Tait bowls badly, Australia's bowling comes right back to the field, not only because Australia lack a third quality seamer, but it also puts extra pressure on Hogg and the other bowlers. And of course, that makes Australia much easier to beat, batting strength aside.