Jamie Dimon, head of JPMorgan Chase, argues against regulation because, according to him, it lumps in good bankers with bad bankers. For example, via Maureen Dowd,

After the economy nearly atomized in a cloud of cupidity, Dimon became known as America’s least-hated banker. But now the blunt 56-year-old Queens native who snowed Democrats in Washington with all his talk about not lumping in “good banks” with “bad banks” has fallen off his pedestal.

For us humble outsiders, it is hard to tell a good banker from a bad banker, particularly when even good bankers can’t catch a “whale”. This generates Gresham’s Law of Bankers – bad bankers adversely affect good bankers. If I am not sure if I am employing the services of a good banker or a bad banker, I am going to make transactions under an expected quality of banker. Then good bankers suffer as they are pooled with bad bankers.

Screening out the bad bankers via regulation can help the good bankers by creating separation.