A number of news articles suggest that a ban on all proprietary trading may have prevented MF Global from collapsing.** It certainly would have prevented it from collapsing as a result of a $6.3 billion bet on its own account.
(**Note that not even the current draft of the so-called Volker Rule would prohibit all proprietary trading by all entities. Some entities will be allowed to engage in proprietary trading and some prop trades will be allowed in specific instances, i.e. for hedging, to make a market, etc.)
There is no suggestion that the proprietary trading that MF Global engaged in was improper. There is no suggestion that the bet that MF Global made on the European debt was contrary to the advice that it gave its clients or that MF Global improperly hid its investment position.
On the other hand, there have been reports that MF Global failed to segregate client funds from its own funds and may have used client funds to pay operating expenses. This conduct is unlawful. It may also be a red flag of other lax standards at MF Global. Maybe the firm would have failed even without taking large, risky proprietary bets.
UPDATE: Check out this recent article on the issue, which includes my thoughts on the MF Global collapse.