Janskeet said:
Why didn't Vader just use the force to break the balcony/suspended platform. If he can use the force to choke someone, why not use it to break a metal beam apart. Why risk losing his saber?

I don't know. Why didn't he choke Luke? Why didn't he forcelightning Luke? Or why not just Force Push the Emperor into the bottomless pit instead of grabbing him?

Was Vader (correctly) confident that his sabre-throwing skills were superior to Luke's "Catching Flying Ignited Lightsabers" skill?

Maybe the metal was too strong given that Vader appears to be significantly less powerful with certain force powers than he had been before being mutilated?

Maybe he figured that since Luke could do a 20 foot backflip onto the platform, he was nimble enough that the platform falling wouldn't be a big deal.