That's also my understanding of their pitch; have we seen any evidence it's actually happening?
Anecdotally in Seattle it is absolutely happening. My small business is one of many that is expanding into these short-term spaces due to flexibility. Who knows if it's actually working well, but I've seen many "sub-offices" at the co-working space where I lease two desks for my employees.
This isn't true for their expanding customer base: Businesses who are paying for blocks of co-working memberships to expand through services rather than finding new leases themselves. There is significant lock-in when this occurs, as people may move close to those WeWork locations, arrange their life around it, etc.