^
^^Exactly. I wasn't implying that Brits and Kiwis and Aussies will be more nationalistic and wish to see a film that included their countrymen (although that may be true to a small degree).
The reason you gave is why international actors bring in a larger international audience. It's because these actors are extremely popular in their native countries. Like I said in an earlier post, Patrick Stewart and Tom Hardy are certainly British, but that didn't bring the Brits out in droves to see 'Nemesis', simply because they were not "hot" stars.
An American actor who is big enough to be well-known internationally is probably to big (and expensive) to play the supporting roles. If they hire someone like Simon Pegg, they attract the attention of U.K. moviegoers without paying for a HUGE international star.
If Abram's cast a young relatively known American (or Canadian) of Pegg's stature for that or another role, it's very possible the British and Australians will have never heard of that actor, even though many American movie-goers have -- and that won't help Abrams attain his goal of creating more fans.