The last reason for keeping your day job while you start your writing career is standard of living. We all have as unique standard of living as personality so you must consider this when contemplating giving up your day job.
For example, I've always worked where I can easily (sometimes too easily) indulge in a "fancy" coffee drink daily. Not only does my day job allow the funds to do this but the means...close proximity. However, I live in a rural area so if I quit my day job I also give up a daily mocha, cappucino or frappacino. Why? Because my writing income may not allow for one along with the fact that the closest coffee shop is 12 to 20 miles away and I wouldn't be able to make that trip daily. This may be a simple example but applies to many things, like lunch or dinner out, expensive make up or spa visits. Any luxury item falls into this example.
Other than advances, your writing income consists of royalites that are paid quarterly. Will you be able to wait to make purchases? Go out for lunch or dinner? Get your hair done?
All jobs changes affect your standard of living and writing full time from home is no exception. You need to look at your lifestyle and see if the life of a freelance writer meshes with your standard of living.