[elengil]

The ring had to be destroyed *not only* to keep Sauron from recovering it, but because so long as the Ring survives, so will Sauron, and as we all see at the battle before the Morannon, Sauron's forces were still mighty enough to have destroyed all of Middle Earth even without the Ring. He simply had the man-power.

The Ring had to be destroyed in order to once and for all destroy Sauron, and it was only by the destruction of the ring (and the convenient rescue by the eagles) that the army before the gates survived. Otherwise Aragorn, Elessar, Estel, Elfstone, ETC would've ended up dead too!