Use cases intro: say that use cases demonstrate why SW - SW comm. is necessary

This change is in response to Chris' comment:

"The use cases still feel like they need stronger justification as to why they're not just iframed SWs.  I know the reasons are there, but it's not immediately apparent to a Bear of Very Little Brain such as myself."

@jyasskin and Chris (I'll send an email to cwilso@), my hope is that this added section primes the reader to see the areas where a SW <--> SW communication implementation is necessary/best. Do you think this properly addresses the concerns above?