Price, Porcello, and Sale could all pitch lights out in the postseason, and we could still lose.
The argument that 'the playoffs are a crapshoot, therefore we don't need Sale' might be a weak argument if we weren't already strong contenders and if that deal didn't deplete our farm system even more than it had already been. You can't look at that trade as just a short term improvement. You have to weigh the short term improvement against the long term ramifications.
Is it worth it? If we win a World Series in the next two years, then yes. If we don't, then no. And that being said, it's quite possible that we would have won the World Series even if we didn't trade for Sale.