I concede your point here. I was about to edit my post to mention this, but I'm at work and my break was over, so I'll clarify now.
The discard only kicks in when the set you roll is...low! This prevents low sets being played
without making the sets you roll that
aren't low any better than if you didn't have a discard rule.
However, if you set the discard threshold so high that it not only eliminates
low sets but also eliminates
medium sets, then this
will mean that the PCs you play are higher than you would expect for that method.
As a specific example, take the 4d6k3 method, with its average expectation of
16/14/13/12/10/9, and re-roll any set without any stat of 14+ and any set whose modifiers add up to less than +1: this rule doesn't touch the average expectation of 4d6k3, so we should expect the same medium and high sets as we would without a discard rule. It's just the low sets that have gone.
But if you set the discard threshold to
higher than the average expectation of
16/14/13/12/10/9, then you
must play higher than average sets.