Iron Tail has 75 accuracy, and you need to sacrifice a moveslot to put it on when Salamence could be running Roost, D-Dance, or Defog, all are really good moves that yoi have to give up.
A coverage move is still a coverage move. It allows it be beat its counters in fairies with relative ease. Focus blast and Iron Tails accuracy issues are outweighed by the pros of beating mences would be checks. Salamence doesnt have to run coverage for every single mon in UU. It can pick its coverage move and let its teammates handle the one or two mons it cant.handle. for example,
Dragon/ground coverage leaves Bronzong and bulky fairies as checks. So Nidoking with Fire Blast is a great partner.
Dragon/steel coverage leaves it walled by Steels, so Krookodile and again Nidoking are great partners.
Scarf moxie sets are usually trolled by physical walls, draco meteor and fire blast bypasses this issue.
The versatility of mence puts the user in a greater advanatage than the opponent, where the user can literally choose the the mon mence cant handle.and have a teammate take care of it.