Have you ever made a tiered cake? They're not easy to make at all! Baking, putting together, and decorating a wedding cake is not a task I would want to take on for my wedding. Plus, you would need to do all the work just a couple days before your wedding.
If you're a professional baker, than you might be able to take on your wedding cake. But I still wouldn't recommend it because it's going to be a lot of stress just a couple days before your wedding day. Also, you will have to worry about transporting the cake to your venue the morning of your wedding when you should be getting pretty.
Some venues will not allow you to bring in food items that were not made by a professional caterer. They have this regulation so that people don't sue them in the case of someone coming down with food poisoning. So, unless you're a professional caterer, your venue might not even accept your cake.
Final thoughts: Leave the wedding cake to the professionals! A cake baker is a need!