Your argument does indeed seem pretty reasonable under this assumption, but I don't think the assumption itself holds. For instance in my investment in bustabit, I am using some offsite -- and when the whale was winning, I started moving money from offsite to onsite (by depositing). I didn't even want to risk the possibility of a margin call (as it would suck to be an investor during all the downfall and then potentially miss out on a recovery). And now the site has (more than) recovered, I have taken that money back out of the site (to lower my CP risk, and keep bitcoin where I feel they are most secure).

Further more, that's exactly what the offsite system is designed to do. You can (mis)use it as a leverage system, but it's going to have ugly properties.  I think in a perfect world bustabit would have both an offsite system (like it does) as well as a leverage system (where you can state your max risk %)