Diana Fox wrote an interesting post on agents with day jobs. You should all go read it. It's very illuminating.

Something else to consider in addition to what she's written:

Agents only make 15% of what you make. Since publishing pays authors very very little and agents take 15% of that very very little pay, do the math.

Furthermore, I think it was Donald Maass who said that it's extremely stressful when a client quits his day job too soon and begs his agent to sell anything so he doesn't have to get a job that gives him steady paychecks. Unfortunately the client is too depressed and/or worried about money that he can't produce his best work, and his agent is reluctant to shop substandard projects.

Now reverse the above situation. Imagine you have an agent who quit his steady paycheck job too soon. He realizes that he can't survive in the commissions he makes, so he tells his clients to pump out books faster so he can start shopping projects around, hoping to get commissions faster even though the client projects could've benefited from an extra month of revision. Or worse, he may get tempted by the client money and embezzle. (Desperation can make people do stuff that they would never dream of doing otherwise.)

I'd rather have an agent with a day job who doesn't pressure me to write faster than I'm comfortable with and / or doesn't get tempted into doing anything questionable / unethical.