Nothing lets you dig as far into your deck as Sage's. Except Juniper.
Speed decks prefer Juniper because they will often play out the hand, then draw 7.
Decks that need to get specific card combinations in hand early to set up (like Tyram and Plume decks) can't always just discard what they are holding, or shuffle it in with PONT. They might have the Typh, but need the Candy. Sure they could play Cheren and hope to get it in 3 cards, or they could improve their chances by playing Sage and hope to get it in 5.
With Bianca, you need to play your hand down to 2 cards (including Bianca) to get the 5 cards draw. Not always possible.
Discard isn't so bad in this format anyway. In fact in can work in your favour with Junk Arm (the Catcher in your discard is now searchable). We also have Super Rod now to get back Pokemon/Energy. You can bear that in mind when deciding what to discard.
Most decks will play a mx of draw and shuffle draw Supporters anyway, and Cheren can be a really good card to have in your deck.