I say go for it, they're probably doing it for the love of their music as they probably don't need the money.
There's a big enough fan base out there to sell out all over the world probably 4 times over.
All music is relevant to some people regardless of it's age or genre, I like what I like and what I don't like I genuinely try to understand it, but respect it if I don't.
I never saw Zep in the hay day, but this could be a chance for me to see them (although I have see Plant & Page).