Actually, unless you are referring to two different scenes, Gandalf does not say that the battle hangs in the balance. He does say: "but if I do [leave], then others will die, I fear. Well, I must come, since no other help can reach him. But evil and sorrow will come of this. Even in the heart of our stronghold the Enemy has power to strike us: for his will it is that is at work.’"

The "battle" that you may be referring to is the one in which Eowyn and Merry confront the Witch King and defeat him, which Gandalf apparently thought to play a part:

"Beyond hope the Captain of our foes has been destroyed, and you have heard the echo of his last despair. But he has not gone without woe and bitter loss. And that I might have averted but for the madness of Denethor."

So as for Gandalf's usefulness on the battlefield, to him it is really moot at this point. The Witch King would have sought to route the Rohirrim and in doing so turn the tide of battle against the defenders, and thus grasp victory. However, Eowyn and Merry defeat the Witch King, and thus Gandalf is not needed to actually take part in the battle more so then he had before.

: So why if as he said the battle was still in the balance, why didn't he go down to the battle? He still could have been useful down there. he could have saved Halbarad, who in my opinion is the LOTR equivalent of the guy in the original Star trek who beamed down with Kirk and Spock (usually wearing an orange uniform) and you just know he's going to die....

Ummm, actually the shirts were red (they were later exchanged with the gold shirts of "command" in Next Generation . . . no one has ever explained why they made that exchange, since blue remained science), thus the term "Red Shirts" as Trek-slang for the security which always proved rather useless, except as cannon fodder.

I always had an image of a group of red shirts sitting in the "Security Lounge" chain smoking and sucking down coffee stale coffee from grey-colored styrofoam cups, just waiting for the intercom to go off and request one or two of them to meet Kirk in the transporter room. Then they would draw straws or cast lots, one guy would hang his head in dispair, the others would clap him on the back, shake his hand, and hand him a phaser belt, for all the good it would do for him. ;)

-RR