My theory fits well, enough....
The Weapons are there to destroy all life on the planet....and turn it into lifesteam, for the planet.
It's a last resort of the Planet's defensive system. The concentrate of Lifestream would make the Planet stronger giving it a fighting chance against, Jenova.
Being that the planet withered all plant life for it's energy, the next step would be to take Life Energy from somewhere else. Something that it couldn't effect directly.... Such as living things..... Man.
I got that conclusion from what is said in those tapes in that Icicle Village. But get what you want out of it. All I can say is that it fits. But it's all speculative.
Gast:
"Do the Ancients, rather, the Cetra, have special powers to heal the Planet?"
Ifalna:
"No, it's not that kind of power. The life force of all living things on this Planet becomes the energy. The Cetra tried desperately to cultivate the land so as not to diminish the needed energy..."
Gast:
"Hmm, even here so close to the North Cave, the snow never melts. Is that because the planet's energy is gathered here to heal its injury?"
Ifalna:
"Yes, the energy that was needed to heal the Planet withered away the land... then the Planet... The Planet tried to persuade the Cetra to leave the Knowlespole, but..."
-----------------------
Ifalna:
"Yes, Professor. The one the Professor mistook for a Cetra... was named Jenova. That is the 'crisis from the sky'. The Planet knew it had to destroy the 'crisis from the sky'... You see, as long as Jenova exists, the Planet will never be able to fully heal itself."
Gast:
"Back then, Weapon was a weapon the Planet produced of its own will?"
Ifalna:
"Yes, but... There is no record of Weapon ever being used. A small number of the surviving Cetra defeated Jenova, and confined it. The Planet produced Weapon... But it was no longer necessary to use it."
Gast:
"So, Weapon no longer exists on the Planet?"
Ifalna:
"Weapon cannot vanish. ...It remains asleep somewhere on the planet. Even though Jenova is confined, it could come back to life at some time... The Planet has not fully healed itself yet. It is still watching Jenova."
Gast:
"Where is Weapon?"
Ifalna:
"I don't know...... I can't hear the voice of the Planet well... Times... have changed. The Planet... is probably watching this situation closely."
Under, secret scenes:
http://www.kuponut.com/index.html?ilogo.html&ilinks.html&http://www.kuponut.com/games/ff7/