You kind of hit the nail on the head there. Aging rockers still trying to sing often comes off poorly, and I find that I am far less likely to watch those live DVDs more than once, which is why I stopped buying some of them.
For example, Peter Gabriel's Growing Up live DVD is nearly as awesome visually as Secret World Live (which is one of my favorite live DVDs by anyone), but his voice is far better on SWL (since it is a decade earlier), so I will almost always go for that one, unless I am in the mood to turn on Downside Up just to be awed again.
His voice isn't bad on GU by any means, but it sounds a bit strained more often than not, while it still sounds stellar on SWL.
And despite the awesome set lists on both, I almost never watch the last two Rush live DVDs, because Geddy's voice is too difficult to take for any duration.