They matter in Kerry's case, as Republicans often say, because he made his military service an issue. It's fair game for research and comment. Even if most of the research has been bogus and the comment big fat lies, I still think it's reasonable to roll one's eyes a bit about the 3 Purple Hearts.
Bush's service--or more precisely, his avoidance of it--is at issue because he has made service to America a major issue, has insisted upon a helluva lot of flag-waving, and because of the cavalier way he's treated groups like the Guard.
To cite one example, we now see that a guy who went into the Guard to stay out of Vietnam (and didn't even really complete his obligations there) has sent some Guard units to Iraq/overseas TWICE, and has brushed off any criticism of these actions.
And, oh yes--this is also a guy who's made old-time values and old-line Christianity major national issues. Are we supposed to just ignore it when the evidence brings out the fact that he--like Henry Hyde, like Strom Thurmond--doesn't seem to have had any values?