The problem is if a small group of MN owners are able to pass proposals that benefit themselves but not Dash -- skimming the cream off the top without spilling the milk. It's the way insiders and parasites have always operated.
The MN voting system is a terrific innovation, but the MNOs have to be vigilant, active and smart not to allow the system to be abused.
I still don't think there is a problem. If some small cabal of MN owners votes for proposals that benefit themselves but not Dash, that is out in the open for all to see. It's not like the proposals are secret - we know where the block rewards go. We might not know exactly who is in the cabal, but we'd see block rewards going out with nothing to show for it. People would lose confidence in Dash and eventually it would die. Problem solved.
Masternode owners -whether they own 1 MN or 1,000 - are incentivized to vote for proposals that improve the network and raise the value of Dash. A small cabal might be able to get away with a few small insider proposals, but eventually it would catch up to them.