First of all i am not arguing against Jirachi being in S Rank and i don't know where you got that from. Second, usage has no bearing for the Pokemon that you are using to cover threats, only for the Pokemon that is the threat. When building a team and you need a check to Jirachi, you can slap SpD Scizor on the team and be fine. Why would you care about usage anyway? SpD Scizor is a viable set in OU with the right support and this is what matters. However, when looking which threats your team wants to cover, this is where usage matters. You want to have the most used threats covered, not obscure sets and Pokemon.