Okay -- into the silent void I shall leap...
>>[Chapindad] Can you join USYSA and US Club at the same time?<<
Yes.
>> [Chapindad] Why would it be important for a club to join?<<
Oft quoted reasons I've heard include lower registration costs [if you're not dual registering, of course], an alternative to the USYS ODP process, an alternative national championship path, the ability to move players between teams dynamically, and less bureacracy [e.g., you don't have to apply and pay for permission to cross state lines and the like.]
For more information,
http://www.usclubsoccer.com/faq/
