For reference, I believe that discard is bad because:
lands - discard is awful
elves - discard is awful (brutality gets a bit of a pass)
DnT - discard is awful (therapy is fine, but you'd never board in thoughtseize)
storm - discard is fine
show and tell - discard is fine
shardless - discard is awful
aluren - discard is awful
eldrazi / dragonstompy - discard is unplayable
delver - discard is questionable, but usually not worth it over other sideboard options
If Brainstorm had been banned, you might have a compelling argument. As it stands, with the upcoming expected format, I really don't think that discard is worth sideboarding over permanent-based hate options that can flex between combo decks.