I don't care that Bush found a way to avoid going to Vietnam
Let's be clear about this. Whether or not Bush used the Texas Air National Guard to avoid service in Vietnam doesn't bother me. If I had been around then I probably would have used whatever resource was available to me in order to avoid service (though, I suspect my reasons for doing so would be different then Dubya's). And the fact that Bush blew off his commitment is not, in and of itself a fatal mark against him. He himself has admitted that he used to be irresponsible. I believe that people can still become good people and achieve great things even if they fuck up early in life. Everyone deserves the chance to prove they aren't a habitual loser.
What Bush did or did not do 30 years ago is not the central issue in this story. What is the central issue is what Bush has done in the last 4 years with respect to this story. The fact that he screwed up back then wouldn't be relevant now except for the fact that he continues to deny, against all the evidence, that he ever did anything wrong.
Compounding that is the fact that he is allowing the dissemination of smears against a man who did serve honorably while at the same time denying that he ever did anything wrong.
This is a relevant story now not because it demonstrates that the Bush of 30 years ago was unfit to lead this country. It is a relevant story because it demonstrates that the Bush of today is still unfit to lead this country.