They shouldn't. They are not over leveraged like the banks were (I don't think they can have any excess leverage). By comparison, retail b/d's are conservative in nature. The only thing that will sink them is if they have a few customers who margin to the max and you have a black swan or something.
The best financial ratings that I have seen for banks, insurance companies and stock brokerages were done by Martin Weiss and company. Weiss was the most predictive rater in the ratings business, where a C was common and A really meant high status. Weiss sold out to TheStreet.com and the bank and insurance safety ratings continue under TheStreet's imprint. The last brokerage safety report books were 2004 based on end of 2002 financial & SEC data. Many of the safest brokers have since been bought or merged by bigger, less well rated outfits. It would be great if someone used that Weiss methodology to comparably grade the current active brokerages. Most of the Big Names that the average person hears were B's or C's, with Lehman and Merrill being C- in Weiss 2004. IB and Siebert were A's then. It would be great to compare the current crop of active trader brokerages here, and their underlying clearing houses to know how well they look today and might do on a quick trip to Dow 4000 or 5000.