I always thought that they knew it wouldn't work. The only thing that they knew would stop Meteor was Holy. Therefore they needed to kill Sephiroth, who was stopping Holy from destroying the Meteor.

On another note, I know they thought Shinra was wrong taking the Huge Materia from the planet, but once extracted why would they need to take it from Shinra? Very good question 'Forsaken Lover'. Does anyone wish to enlighten us?