Man, you guys just don't give up unless somebody can quote you chapter and verse that proves otherwise, do you? Lucas has referrred to Vader as being somewhat of a cripple since at least the mid nineties. Its either in the introductions to the THX remasters or the Annotated Scripts (or both) but I don't recall which and I'm too busy to look it up. It is certainly in the Making of Phanton Menace, where he refers to the saber battles in teh OT as being fought by an old man, a child and a half-droid. And besides that, in your Vader-worship you're missing not only the point of that scene but possibly the entire Prequel Trilogy. Mechanical Vader cannot be seen as any sort of reward. Its a prison that he has to remain in for the rest of his life. It would ruin that if he somehow retained all of his physicality, instead of being far less of the man he once was.
Emperor
It seems that in your anger...YOU killed her.
Vader
I...I couldn't have. She was alive! Noooooooo!
Emperor
But on the plus side, I had them build you this
nifty mechanical body.
Vader
(flexes arm)
Impressive.
Emperor (ctd)
And they whipped up this nifty looking suit
that I'm afraid you'll have to remain in.
Vader inspects his image in a mirror.
Vader
Padme who?
They laugh maniacaly.