It is the equivalent of building a house without blueprints. You can do it, but if you take the time to plan things out before you break ground you will end up with a stronger product.
Now, if you already have a design style locked down and code you can just puzzle together then sure, go ahead and skip Photoshop. I do that as well. But, as soon as I need to design a new UI element I am back in Photoshop making dozens of variations.