as "Is BitShares vulnerable to attack in situations where BitShares holders will lose money", then having them place bets results in a more similar scenario, and more similar game-theory / socioeconomic climates.
The way I see it, Bitshares holders are in a constant state of betting that Bitshares will not suffer from attacks (as an attack would decrease the value of their holdings).
But if they also make an extra 'bet', they are in essence subsidizing any possible attack.
For example, if I can attack Bitshares for a cost of $X, and you offer my a bet of more than $X if I successfully attack, then you have simply opened yourself up to a vulnerability. I take your bet, spend $X attacking, then collect and make a profit.
Bitshares best defense against attacks, imo, is that they not only cost money, but the money ends up in the hands of BTS holders and/or traders in the end. Either through a failed nothing at stake attack, where the attacker loses money due to slippage when buying and selling BTS. Or due to the scenario in case of a severe attack which Bytemaster has described, in which the community responds by hard forking Bitshares, cutting out the malicious stake, and proceeding on as before, but with a lower effective BTS supply. In the end, humans play a role in the Bitshares consensus algorithm.
I would not want BTS holders to make a bet that Bitshares can be attacked, because this might subsidize the attack.
I would like for the attacker to lose money on the attack (whether successful or a failure), and that money go to BTS holders in some way.
I believe that Come_from_beyond could disrupt Bitshares, given a willingness to lose enough money. I just don't think he can do it for cheap.