Quote from CloroxCowboy:
Obviously you would say that the diversification benefit outweighs the cost of having to fight for a good fill on SPX. I'd have to try it out to see if it would be worth the hassle for me.
A more objective question might be: Does the diversification benefit outweigh the losses on SPX over the long term if the trades were made at or just inside the bid/ask? IOW, if I could see even a minor gain over time by diversifying without having to fight the MMs too hard, it would be an obvious choice for me.
I would never pay the full bid/ask spread on an IC in any index!
I wouldn't either, unless the benefit from owning multiple indexes had a statistical advantage large enough to offset fill loss. Even then, it would probably make me FEEL like I was losing every time, so it would have to be a large advantage.