The fourth reason to keep your day job is benefits. Now, I did talk about health insurance, which is a benefit but there are also other benefits to consider besides health, life, dental and vision insurance.
One reason is taxes. Employers withhold and report your Medicare, Social Security, Federal and State (if applicable) to the appropriate government enities. Do you know that your employer matches the withholding on Medicare and Social Security? So if you are writing full that means you will have to pay both your withholding and your match amount. If you are not savy with numbers or accounting priniciples you'll need to hire an accountant to report these taxes to the government. Paying taxes will require you to put aside "X" amount of money from each advance or royality payment so your taxes can be paid quarterly. This is in addition to any sales tax you need to report from direct sales of books.
The second benefit is retirement savings. Many employers offer a 401(k) or 403(b) to their employees. Many times they match up to six percent of the what the employee saves per year. Six percent is free money to the employee! And that can add up over time. Plus these benefit programs can make it difficult to withdraw your savings so the money will be there when you retire. Yes, there are retirement plans that a freelance writer can invest in but statisics show that most people are not good savers. If given the choice between sending money into a IRA account or splurging on a new tv, many will chose the tv.
Just a few more things to consider before quitting the day job.