One of the most profound errors in the Bush Administration was in not using Cheney more -- he could have and should have been the spokesperson for the Administration. He could have been very effective at something that W just had no confidence and no competence. I'm sure the Administration's leaders decided they couldn't let Cheney have that kind of public and prominent role because it would have fueled the suspicion that Cheney was in charge. So W's Administration just went ahead without any effective communication to the American people defending the Administartion's policies and the long-term vision they were following. Compare W's success with that of Reagan -- and I think this inability to communicate is by far the primary reason.
Having said that if we say he still was not used enough maybe we go back to the loyalty aspect with the President. VP Cheney has always said that his role within this administration is promoting the President's policy and he would not use it as many VP’s have for furthering their political standing or career. He made it clear that at the end of either the 4 or 8 years that was it for him in an official political/governmental capacity in Washington. I am out of here is a term he has used often.
Bearing this in mind this would therefore the President feel happy in using the VP as a spokesman for some issues with which they did not agree? The marriage amendment springs to mind even that though Cheney the loyal servant he is did promote it a few times even though privately it is known both he and his wife felt it was a State's issue not Federal one. No doubt this is partly to do with sexual orientation of his younger daughter Mary but that aside he was still prepared if push came to shuff to support the President.
I would suspect there were other issues probably one of them is in how to tackle the immigration issue where he and the President did disagree again here there are few speeches where he did push the President's views and wishes but compared with other issues I agree he has not been that outspoken on this probably because privately he did not agree 100 per cent with the President.
Now going back to loyality aspect I would suggest the President would not feel comfortable getting the VP to constantly go against some of his own personal views to promote the President's point of view and that is why I suspect that maybe he has not been used on some ocassions as a spokeman for the adminstration. Again you can say the President maybe was wrong and that as the Commander in Chief he should not have taken this into consideration but I believe that this is part of his character and his own loyalty that he would take it into consideration.
One footnote from me on this take this away from the President and you take away his character and what IMHO makes the man tick and he would be a lesser person maybe not a lesser President but certainly a lesser person.