Hyra said:
Obviously dual STAB is preferable, except on Gengar, Heatran, Lucario, Salamence, and Gyarados. Hm... all are top 10 sweepers who only use one STAB move. It's not necessary to have a dual type, and in this case, most of the dual types would outshine Rock.
You're missing another extremely important point, which is the fact that while none of those Pokemon may use dual STAB, they do gain a lot of benefit from their dual typing in terms of resistances. Gyarados wouldn't be nearly as popular if it were pure Water, since it now can't come in on things like Heracross or Lucario. Same goes for Salamence. Heatran is pretty much made by its dual type; without all those valuable resistances, it'd have a much harder time switching in safely. Gengar really doesn't gain a whole lot of benefit from its second type (mainly since Ghost is already immune to Poison's most useful resistance, Fighting), but the resistances it provides never hurt.
And of course, a lot of sweepers DO use dual STAB if granted it. Tyranitar, Heracross, Garchomp, Infernape...there are a lot of sweepers out there that wouldn't be nearly the threats they are now without having two STAB attacks.
Not only that, but our Pokemon is most likely going to end up a fast, frail sweeper. On a defensive Pokemon, more resistances/weaknesses aren't as important since it's already capable of switching in on account of its defenses. But on something fast and frail, it's going to want as many opportunities to switch in as it can get. Now, is switching in on Normal, Fire, Poison, and Flying moves preferable, or would you rather be able to come in on Dark, Rock, Bug, Normal, Fire, and Poison attacks? Not only that, but Rock's list of resistances is pretty terrible as it is right now; it almost needs a second type if it's going to be switching in frequently with any degree of safety. A second type is a huge boon to a Pokemon like this in both terms of offense and defense; it's kind of silly to say that our Pokemon doesn't need it when it's one of the best things we could do for it!
And this isn't as big of a deal, but pure Rock doesn't really do much from a design perspective. But combine Rock with something else like Fighting or Dragon, and suddenly the design possibilities become a whole lot more interesting.
