Actually there is one thing that I'm not sure about. Who votes with the collateral for MPA? Is it the person who created the MPA or who owns the MPA? I assumed it's the one who created the MPA – it's his collateral. But I realized I'm not 100 % sure about this.
If bitBTS is used, it would go like this:
Somebody creates bitBTS, backing it with 100 % BTS collateral. Nothing has changed, he has same amount of voting power. The he sells the bitBTS for real BTS. Now he has voting power for the real BTS he just got plus the collateral for the bitBTS that he created. With this trick he can double his voting power without any change in the value of his assets.
DAC could incentivize the creation if bitBTS by setting the creation transaction free. Settlement would have a cost so there is no possibility for spam by changing BTS to bitBTS continuously. It would also discourage people to change bitBTS to BTS just for fun.
Of course there is not much point of doing this for Bitshares now. BTS is so widely distributed that it would require an awful lot of work to get it changed to bitBTS for all of those people who are not interested in actively taking part in DAC decisions.
But if somebody was going to create a new DAC, this might be a great thing to do at the beginning. Original investors would hold the core asset and they would sell only derivate of core asset to everybody who is not interested in voting (most of speculators and traders). This way the original investors would keep the voting power for themselves so the voter apathy problem would be very small.
Speculators and traders would have an incentive, although possibly quite small, to hold mostly bitBTS. If they are not interested in voting, it's better to let those people keep the voting power who are truely interested in it.