It is and I would love to see it addressed.  
It doesn't have to be an exchange either, it could be hand full of whales joining forces given that there is a financial incentive.

In theory, if a person/group has the ability to control enough weight to grab a delegate spot bellow 100, than any sit above that is guaranteed.

However, if they are forced to split the weight, this would decrease the likelihood of such scenario.

Or am I missing something? Lets discuss. I think the last offical comment made on this kinda brushed off the issue with the suggestion of possible dapp that would help midigate this concern.