Is this false economy?, or good sense? 

Depends why you are replacing them.
If they have bits missing, tree sap or frozen to glass damages the edge, then change the rubbers.
Poor wiping due to weak spring (center poor but edges good), then change the whole arm. Tangiers Orange - gone, missing her
Replaced by Ewok what a mistake - now a happy Disco Sport owner

can you do this on modern one piece wipers or are you refering to the rear wiper?

if so just buy new one At work
At home

"I can't always believe facts I read on the web" - Charles Dickens

winner by default of the tractor vs caravan race

pcheaven: you have pm!