I like to own the emerging markets index mainly because over the long term, in my mind, it should be a higher volatility, higher reward index, and I feel aggressive enough to get a piece of it at least.
I see US stocks as safer in general, and hence in the long term shouldn't they provide a bit less of a reward? I'd think a super aggressive investor might be totally focused on developing countries for the higher risk, higher returns (of course there are other barriers like taxes etc. which could be the reason most people don't, but theoretically). BTW I think higher risk is really fraud/political in nature, not due to any inherent disadvantage of a foreign company. Either way it's like lending club in a very general way - you make the most money off the people with the lowest credit rating.
As for international exposure - yeah if the international exposure of the US index isn't enough for you, then sure, get some more directly because these days, the management fees for doing so aren't too bad. Everything in moderation. Don't see any reason why US stock portfolio should always win in the long run, that just seems racist.