In "A Colossal Failure of Common Sense", Patrick Robinson provides a pretty good example of a pretty useful thing that investment banks appear to do - providing liquidity in the market for distressed corporate bonds.
He gives an example of some airline that announces it has financial troubles - immediately people holding the bonds they issue are pretty keen to sell, pretty much at any price. The bank have had very smart researchers who spot that the airline looks like it will be in trouble and then estimate how much the airline is really worth and then buy at below this price (based on their assets - planes).
The people offloading the bonds are presumably pretty happy as they are worried they might be worthless and the bank is making some money off of the risk that their research might be wrong.
Sounds like a pretty fair way of making money to me.