I know of at least one quest where the weight your carrying adds to the plot (Wiz-King).
Medium encumbrance takes you off center as a monk.
I would not mind the portable hole section of the backpack being weightless.
But taking the weight out of the game all together would be to much of a game-breaker, imho.
(I say this while I do know it would help a lot of dex over str builds.)