As one source puts it, "[i]n a pejorative sense, commingling is the special vice of fiduciaries (trustee, agents, lawyers, etc.)
So in case of bankruptcy, if the funds have been properly kept separate, they can easily be returned to the client.
Attorneys are strictly prohibited from commingling their clients' funds with their own, and such activity is grounds for disbarment in virtually every jurisdiction, because of the ease of embezzlement and the difficulty of detection.
[4] Similar rules apply for licensed real estate brokers handling earnest money and other professionals who hold deposits as agents for clients in absentia.
For small business, strict separation of corporate and personal property is a particular issue, notably in tax and divorce law.