This reasoning misses a very large portion of Polypane, which is that you use it while developing, not just testing. It's a replacement for your time in Chrome/Firefox/Safari, not your time testing across devices (real or through online tooling like Browserstack).
It does however minimise that device testing time. So it all depends on how valuable that time is for you.
It does however minimise that device testing time. So it all depends on how valuable that time is for you.