Agreed. Not necessary. The way prices work on keycaps, as long as the GB meets the price break it's aiming for, ordering to the next price break costs a comparatively small amount. If the GB exceeds its price break by even a few keys/sets, the extras are almost free!
The problem is how to use that fairly. Distributing the gain around the group buyers leaves the organizer with a fairly large risk of being hit with a hefty loss (and piles of unwanted caps). I honestly feel it's best for everyone for the organizer to use extras to make some profit. It gives organizers confdence that they won't face a loss. Buyers still get keys at good prices. In theory organizers could make quite a decent profit, but let's face it, right after a GB the extras are NOT going to sell like hot cakes!!
There was a fairly long and open discussion about the economic side of it in i3oilermaker's billiards GB thread.
As for the political/moral side of it, I strongly disagree with those that demand there is no profit from a GB. That just puts the organizer at risk of a loss, and they won't do it.