A couple of things here. Clients find MPs in secondary sites using boundaries in Content Location boundary groups. I know that sounds odd, but Site Assignment Boundary Groups are for Primary Site assignment only. Also, client's are never really assigned
to secondary sites so it wouldn't/doesn't make sense to use site assignment boundaries for secondary sites.
Also, "some" client communication to the MP in the primary is still required even with a secondary site and an MP at that secondary site in place (note that in 2012 having a secondary site without an MP is impossible). Secondary sites are *not* gateways.
For reference: http://blog.configmgrftw.com/?p=453
Jason | http://blog.configmgrftw.com