Acording to IGN, each "story" (Leon, Chris, Etc.) is something you play through entirely from beginning to end without shifting to another. Though you would obviously be missing out on part of the game, it seems quite possible for you to entirely skip Chris's portion of the game if you so choose. But I'm betting you'd feel compelled to play it just because it's there and then say how much you don't like it being there... Essentially giving Capcom no way to win here.

Personally I think it's very smart, they're giving content for all different fans of the series. In theory, it makes a lot of sense at pleasing everyone (or most everyone, you can't ACTUALLY please everyone when there's this many people involved). The only way to know for sure though is when the game gets closer to release and we get a lot more info.
Oh, and IGN also said the blonde girl in the trailer was Sherry Birkin.
My big question for this game though is the timeline. Supposedly it's 10 years after the Raccoon City incident, which would place it at 2008, I believe. RE5 was 2009, so is this game actually before that game? I'm hoping this isn't the case and this gets cleared up later.