No. Different auras stack, so the defiance aura is in effect all the time, along with any other aura. The aura graphic rotates through the different auras in effect, to show players what auras are active.
EDIT : Unless you mean a paladin that was using a higher level defiance aura - then it would alternate with Exile's defiance aura (I believe). I can't immediately think of any Paladins which use defiance as a primary aura (Cleric?), but I don't think it's a significant issue for paladins in general.
EDIT : Unless you mean a paladin that was using a higher level defiance aura - then it would alternate with Exile's defiance aura (I believe). I can't immediately think of any Paladins which use defiance as a primary aura (Cleric?), but I don't think it's a significant issue for paladins in general.