How many do you need?

I was listening to the new Exodus album the other day and although it is good, I had to ask myself how many original members should a band need to use that band name? Exodus did a comeback album last year and they had three original members plus Steve Souza who had played on every previous studio album except the debut. Now on their new album, Gary Holt is the only original member left. Although good, the album does not immediately sound like Exodus. I know that using the name Exodus would sell many more albums then if Holt were to use his name or a new band name, but just using a band name bothers me in some small way. I know that as a consumer that I have the right to not buy it, but there is just a part of me that believes or at least hopes that a band should mean more than just being a marketing tool. Of course I am probably just being naive if I believe that.
One of the prime examples of one member using a name is Black Sabbath and what Tony Iommi did in the mid-80's. Seventh star was released in early 86 under the Black Sabbath name although I believe it did say Black Sabbath featuring Tony Iommi. Really it was just Tony and a hired backing band. When asked about using the Sabbath name, Iommi said he wanted it to be a solo album, but Warner brothers insisted on him using the Sabbath name because it would be more easily recognized. Then Iommi continued to do several more albums years using the Sabbath name.
A recent example that relates to this discussion is Megadeth. The band broke up after mustain had health problems. Then after a brief absence he decided to start the band up again only he was the only original member as bass player Dave Ellefson was not in the lineup. This left Mustaine as the only original member and some people wondered should this really be considered Megadeth or was it just a Dave Mustaine solo project by this point? Should using a band name mean more than just using it because you have the legal right to do so? I would like to think using a name means more, but I am a fan so my priorities and train of thought are different.
So this would bring up another question and that is how many original members does a band need to use that band name? My initial answer is two because having one is no different than it being a solo project in some respects. I say that is my initial answer or at least was, but then I thought of some exceptions that made me question that answer. Running Wild and Grave Digger are two bands with several similarities. They are both from Germany, both had debuts in 84, both have similar sounds and both only one original member left. In both cases the original member is both vocalist and one of two guitarists in their respective bands. I don't have a problem with either of these two bands using their band names despite the fact that they only have original member. The reason is because I feel that both have maintained a similar sound throughout their careers despite the line-up changes and I think that's the ultimate deciding factor in any of these cases. So ultimately it boils down to the music and not what band name is stamped on it. It can say Black Sabbath on the case, but if it doesn't sound like them to me then I may have a problem with them using the name.