Personally, I WANT a sub, but haven't been able to justify it in my mind. Not for movies, but if you're really into bass, (mid range and treble is more important to me), and want to play music loud, go for it.
One thing I love about SONOS, is their return policy, which I have done, so order one after you have the base system set up, and see if it makes enough of a difference to you.
Definitely get two Play 1's or 3s for rear/surround. BTW, I have the Playbar, and they have since come out with the Playbase, which I read sounds as good, but is a bit more versatile. I think the pricing is close.