Another reason to work a "day job" is health insurance. Although many writer's organizations provide health insurance or the accessiblity of health insurance, these premiums are usually very high. You can purchase purchase private health insurance, but again the premiums are high as well as the deductable and keep going up if you have a pre-existing condition.
Depending on the size of the group, the group plan at a day job will usually have lower monthly premiums and better benefits and existing conditions are not factored in. So more bang for your buck. Group plans do vary and there are many small groups that do have higher premiums but probably NOT as high as private individual or family coverage.
If you are not lucky enough to have a spouse who carries the insurance for your and/or your family, you may want to keep your day job for the health insurance coverage they provide. Medical services are expensive and can add up fast which could lead to loss of productivity and creativity.