The fact that he was in the Reserves is not necessarily a negative. The Reserves had requirements for active duty, and when on active duty served just like the Regulars.
The main difference was that the Reserves were mostly NOT "career" Military.
The point is that Kerry did the same as Bush, he volunteered for the Reserves there is a strong similarity there. The only difference is that Kerry got sent, Bush didn't.
The Dems like to paint Kerry as a brave hero who volunteered for the Navy while Bush cowered in the reserves, but the truth is they accepted similar obligations.