What happens if I leave all of my images in the drawable-hdpi folder?

Lyrkan is mostly correct. Mdpi phones will take the image from the drawable-hdpi folder. But the system knows that those images are too big for the phone and scale them down for you.

Most of your users won't notice. The only problem is that on phones with mdpi or ldpi screens all the images will get a little bit blurry because they are scaled down on the fly to match the other screen resolution.

The phone is gonna take the images from where they're available. The result will be the same than if you had only a "drawable" folder, it is just not a logical name if you have only one drawable folder.

You are not forced to provide resources for every configuration, the system just take care of using the most adapted and is clever enough to do that job. You should read that to fully understand how it works : http://developer.android.com/guide/topics/resources/providing-resources.html#BestMatch

You can leave your images in one folder, it would be used by all the phones. The different folders are used if you need to load different images for different devices. In fact, the device will start looking in the folder related to its density, if it doesn't find an image usable, it will look in the other folders. Use the different folders to load different images for different densities. If you don't need this feature, leave all your images in one folder only, no matter which.