✔ 最佳答案
You are allowed to bring in some legendary Pokemon like Heatran, Latios, Latias, Cresselia, ROTOM, etc. into the Battle Tower. I think this is because they are not considered "ubers" (too strong) by the creators of the Pokemon games. I find it strange that Regigigas is allowed in the Battle Tower.
However, Pokemon that are considered uber (Dialga, Palkia, Arceus, Darkrai, Shaymin, Rayquaza, Kyogre, Groudon, Mewtwo, Mew, etc.) are not allowed inside the Tower because they are "too powerful".
And yes - you're going to fight Legendaries in the Battle Tower on occasion, but they won't be ubers. I believe the most powerful you'll see are Regigigas, Cresselia, Heatran and possibly Latios\Latias. The potential for fighting an opponent with legendary Pokemon greatly increases depending on how many battles you've won. The longer you succeed in the Tower, the better chance you have to fight a legendary Pokemon. The Pokemon you use does not matter.