They should have because he has a more appealing voice than them. The reason they didn't is because they wrote the material and wanted to sing lead on what they could handle. They gave Bobby the parts that they could not sing.

To me, they let their ego's get in the way. They did sound good on alot of the songs they sang but when you have an accomplished vocalist in your band, you should let him sing as much as possible.


Another great example of this phenomenon is Simon and Garfunkel.
Paul Simon wrote most of the songs, so he had all the power. Same thing happened in Toto.

Paul Simon is in a band with a guy that sings CIRCLES around him and he insists on singing as much or more for what has to be because of his ego. As great as they were, they would have been that much better if Art had sang lead more.