I've been reading about Wagner tubas and I've been wondering how I can determine whether I want to use it in a composition. I know the purpose of the instrument is to bridge a gap between the timbre of conical and cylindrical brass, but listening to demonstrations has left me wondering why I couldn't just use a tenor tuba or regular horn in F in its stead. Some composers such as Wagner (obviously), Respighi and Holst seem to favor it for certain solos, but to me, it doesn't have much of a notably unique sound. In fact I've heard that Wagner tubas and tenor tubas are essentially one and the same, except that one has valves whereas the other has pistons.
Has anybody scored for this instrument, and how do you compare it to various other brass instruments that sound similar to it? In other words, why favor this one over another, or have it as an extra in the orchestra?