I think Dave has a lot to offer, and he could still front VH successfully if they reunited tomorrow. The problem with many who post about this issue is that many folks never saw Roth with VH live. Many Sammy favorers became fans when Sam was in the band, therefore have a slightly skewed view of Roth to begin with. For those of us who love Dave in VH, we cut our teeth on the early VH records and hold the original band close to our hearts. However, many of Dave supporters have to remember some things. Dave was never a great "singer", at least live. Back in the day, he ran around, drank whiskey, and got most of words right (some of the time). Dave was a great vocalist. What I mean by that is that he could deliver a flavor of vocal styling that was different from the rest and ultimately classic. Whether he can do that today with the same vigor and energy is debatable, and I bet it can be debated on a show by show basis. Dave still drinks whiskey, smokes cigs & dope, and bangs groupies by the dozen. His lifestyle doesnt often permit perfect voice tone and stamina. He didnt have it back in the day (on a regular basis) and he doesnt today.
There is no doubt that Sammy can still bark like he used to. He can very easily jump back in and be successful. But what we old school VH fans miss is what Dave represented, and still represents today to some degree. We miss the ballsy swagger and delivery that makes no excuses and makes for one hell of a show. Whether that still motivates you after all these years or not, listening to a Roth era record makes me smile deep down.
Dave couldnt do Sammy material. Sammy can do Dave material, although he hates doing it and its evident when they do the old stuff live that Sam is dogging it. I'd rather not hear it if Sam is gonna let the crowd sing or not be into it. So there is the catch.
My question is - Do we really need 3 or 4 more VH records with Sam? This isnt gonna be the Rolling Stones here people. Have Sam and Dave do a world tour with VH. Sam set and a Dave set. Separate busses. Record a double live dvd and put out a cd to match. Create one mater anthology and pack it in.
Sure Dave has lost a few steps. Then again, listen to Robert Plant and tell me if he could still sing every song in the Zep catalog the way he used to. Everyone loses a step. Can you still do the same things you did when you were 20 or 25? The fact that Dave is in better shape than any other current VH member shows that he may not be as washed up as many think.
I will enjoy the new VH with Sammy returning. However, when it comes to the live show, I will be quietly longing to see Roth pacing the stage once again. That is my preference only. To answer your initial question: Roth is not washed up and can still bring it. It just depends on how much excess is involved.

"That's why I hang around with two of 'em - if there has to be any conversation, I don't have to be involved!! Forgeddaboudit!!!"