I agree with you. Personally I don't like this hero. Other Mordor heroes are more interesting. Mollock is just too expensive troll witnout voice.
I think that great voice is important for hero. Rogash from vanilla is a much more interesting character with great voice.
I admit I don't like it as well, and I generally tend not to use it
But I also think that these two heroes (especially the Mouth of Sauron) are worthy of their price.
Also, I would say that if one decides to build outposts, instead of further heroes, it's 'just' a different and alternative choice of strategy, depending mainly on the characteristics of your opponents; I don't think that it necessarily means that some heroes are useless or not worthy of their price, as all of them must be used carefully as tools in your hands
My vote, then, is negative.