It seems that a lot of the support for this legislation comes from the misguided idea that banks are allowed to mix investment/retail activities, even if they are under the same roof. That is just not the case. In fact, the highly publicized Volker rule prohibits just that.
The only reason the two are under one roof is for diversity. When the trading arm loses, it can be offset by the retail, and vis versa. It actually limits volatility (as diversifying almost always does).
I would like to hear a serious argument as to why the Volker rule is not enough, and we need to actually have the goverment break up some of these institutions.