In RichFaces 4 there is rich:select which is pure select at server side but allows typing in order to choose value simpler.
As for 3.3.x - comboBox is UIInput component. And f:select* support not native at all for it. The biggest concern that with select you could only select from the list of values and with comboBox you could enter absolutelly everything you want. So we considered that two usecases and inpolemented as I told above autocomplete(ui input) and select(ui select one) for 4 version.
Thanks, that makes sense.