There is a stupid high statistic like 90 odd percent that if you buy one you'll buy another one within 12 months.
Well Sonos is a bit pointless if you only have one!
I guess part of the reason is people buying Sonos are looking for a system with multiple speakers. They get a Play 1 to test and then buy the others after they find they like it. That's what I did at least, it's quite nice to be able to test a system fairly cheaply before committing.