Why should I develop my applications using the MVC design pattern?

As your site grows, it will be easier to manage if your display is separated from your code. You want to modify the design ? It's all separated, you don't have to worry about the code in the HTML. You want to modify the way data are calculated ? It's all separated, you don't have to worry about the HTML in the code.

Your code will also be easier to read and if you want to add other people to the project later, it's easier for them if your code is separated from your HTML.

Once you are used to MVC development, it doesn't really take more time to develop than "normal" development.