But it gets STAB from Earthquake, anyway. Earth Power is a special attack and doesn't benefit Swampert in any way while you've got Earthquake. If you switch Swampert in on a Choice Band, using Earth Power has no point because it doesn't get any boost. :x Superpower, however, is both physical & can provide good coverage.
Aaand Fire Blast because you'll likely end up facing steel types, in which your dragon STABs can't do anything to (aka why they send in steels), while Fire Blast has just enough power to destroy those steels.
Also remember that Bullet Punch has priority, and a lot of the time it's necessary to use. You could always stick Brick Break or Superpower in the place of Bug Bite 'cause you have U-turn anyway (aka bug STAB and allowing you to switch!) in order to take on Magnezone and other steels you can't face otherwise.