Yes, it works for YOU, but what about the person after you? (Assuming you're in a business setting.) There's actually a chance whoever comes after you will have experience with one of the popular frameworks. There is zero chance they will have experience with your custom framework. I inherited a custom framework at my job. I wish the person or persons before me had just used an open source framework. The popular open source frameworks (Cake, Zend, CI, etc.) are better documented and more thoroughly tested.