I get the impression the focus among celebrities is mainly on vegan diet rather than vegan lifestyle. As such, it's just another "miracle diet" in a long row of other miracle diets. And how long can a celebrity stick to a vegan diet? It's almost a joke now.
I can think of one or two honest exceptions, but in general, with regards to veganism, I don't think vegan celebrities have much to teach us boring, regular veg*ns.