Quote from Ghost of Cutten:
The problem is that the more brokers you use, the worse your overall credit risk, and the higher probability you find one of your brokers blowing up. If broker A is the safest by far, broker B 2nd safest, and brokers C though N are a bit iffy, and brokers O-Z are awful, then the safest strategy is have the majority of assets with broker A, and some with broker B.
There's also the execution problem - you really want to have to log in to 2 or 3 systems at once during a fast market, placing several orders each time you want to trade?
The best way to reduce broker risk is to do proper research into the risk policies and culture of each firm, look at the strength of their balance sheet, and look at the stock price of the broker or parent, as well as any potential 'put' from a corporate parent (e.g. Newedge is owned by Socgen - unlikely to go bust as they are TBTF for the French government). Pick your 'best' overall broker and keep maybe 70% of your account there, and then a second broker (also safe) for 30%. Any time the stock gets to distressed levels, wire out most or all your funds, and definitely do so if the rumour mill starts. Any more than that is probably counterproductive.
You should not have 100% of your net worth in brokerage accounts anyway, I would say about 30-60% (depending on your trading strategy and its need for collateral) should be in a low risk portfolio of cash, bonds, stocks etc, held in your own name in a cash account, not a nominee/'street name'/margin account.
Note that in Lehman, Refco, and MF, you had ample warning each time - the stock price chart warned you to get out long before your cash was at risk.