although I never saw them in person so I don't doubt your at all about DLR's showmanship or his worth to the band. MOST people are into music for the vocals and DEFINITELY the showmanship... I'm not, which makes me a weirdo.
I'd put those three in this order: 1: The Instruments 2. Showmanship and 3. Vocals.
I saw few heavy rock or metal bands live. Well.... maybe more than you're thinking but to me it wasn't a 'lot'.... I remember seeing Motley Crue and the 'show' was simply outstanding. I had to rethink things after seeing them... not about MC exactly but about 'showmanshiip'.
And then there is this.... I've never seen any two people command stage presence the way Elvis and Elton John did.... they didn't have to do a THING... just standing there the energy blew the roof off the venue. I think they way you describe DLR makes me think he was the same.