That is a good question. That made me remember an artist who told me during a photo shoot that she couldn’t use models for her painting so she decided to use herself as a model.
And actually, I do the same for numerous reasons. First it’s handy, always available. It's not expensive and usually pretty understanding.
Don’t get it wrong, it's not a matter of narcissism, i’m pretty shy in front of a camera. But of course, there are some downsides you can’t do everything. Selfies are limited in terms of poses and compositions and it does not always flatter you. And if you have a chance to have an assistant it’s not always helping. You have to make him understand what you even when it's confused in your head, and believe it it's not an easy task. And nothing can replace your own eye so you can guess the long irritating session to come.
So pros and cons are balanced at the end. But when you are a broke photographer do you really have a choice? And easily it becomes a habit.