IIRC Luke's hand being severed was meant to be a 'link' between them (seem to recall Lucas stating that in an interview,...not sure)
If nothing else it was done with Luke solely because of that scene, Vader had to end the fight without killing Luke,...yet leaving him powerless and vunerable at the same time. Vader's arm being severed,....well at least it beats the hell out of the "Splinter of the Mind's Eye" version,...were Luke (While possed by Ben) cuts Vader's hand off,...then Vader still manages to best him in combat,...only to drip over his own flippin severed hand and fall into a pit.

Plus,... severing a limb is pretty much the best way to end a saber duel, in EU there's been about 15+ Jedi/Sith that had one thing or another sliced off.