[Břicho]

Well, I guess I'm going to have to beg to differ.
I think it was more than politics; it was an evil that no one else could have averted but Gandalf. Since there was no one else to save him, Gandalf had to do it. Unlike Theoden or Halbarad, Faramir couldn't defend himself.

Beregond would have gone down sooner or later, and he had already killed men(who were arguably also innocent.) Gandalf was faced with a moral decision: help out in the battle and let an innocent man be murdered; or save Faramir knowing that his efforts would be missed in the battle. The battle was going to happen no matter what and people were going to die; but the "other battle" was maybe a greater evil: it had innocent soldiers killing innocent soldiers, "tangled in a web of conflicting oaths..." And an innocent man about to be murdered because of madness.