Okay, you're right about Hermonine. However, keeping in mind she still just a human.
Twilight herself shrugged an anivil, a fridge, and a piano, while already hurt from an accident WITHOUT magic. Of course it would be unfair, especially considering Twilight's "wand" is attached to her forehead.
Also, considering the whole episode was practically Dragon Ball Z, I'm pretty sure Twilight can or could learn to tap into the Princess Power by herself, and learn to control it like actual Super Saiyan. Remember how weak Twilight was during Season 2?
"She may be the most magical unicorn in all of Equestria!"
-Official Enterplay Trading Card.
I don't think Rainbow Power was a one time thing either. The REAL question is whether or not they can use it separately.
The show is already turning into DBZ anyway, so they might as well give it too Starlight, too.