Stability in business
Business' want stability and profits, sometimes companies can put long term stability at risk due to the pressure on surviving in business. This is not necessarily business' fault just how economics works, it's similar to athletes who can put their long term health at risk by taking certain drugs to improve short term goals.

Retail and investment banks
Combining the retail banks and the investment banks adds greater risk to the whole economy if you potentially go under as everyone loses their money, the benefits for the company is using the money from the retail banks in the investment banks what is more profitable.

Any company that decide to combine both sides of the bank can make more profit and make better offers to customers. Even though decisions like these can make you more unstable in the long run, any company that decides not to combine both sides of the bank can be less profitable and is more likely to go out of business, so you will more likely see companies putting their stability at risk to survive in the short term.