The 2DS does have one screen in a technical sense, yes, but that's for cost saving factors.
You asked if they get used enough? Short answer: Of course. Long answer: Of course they bloody do. Every bloody 3DS game uses it. Some to extend the HUD, others for a variety of other factors. Dual screen gaming is the most beneficial "innovation" Nintendo has done in the last 10 years. It also separates the console from the generic single screen dross that plagues phones etc. and gives people reason to see it as a separate device.
Yep, I've been repeating this continually around the net. People, somehow, interpreted Iwata's words to mean a hybrid is coming -_-