If the op is giving you a mini class with your Nitrox dive then you are getting something moderately useful from the deal. If they are just giving you a tank of Nitrox to use, that provides value to a Nitrox certified diver (nothing wrong with a free tank of Nitrox) but to someone without Nitrox certification, I don't think it's worth bothering. While it's unlikely to hurt you as long as you don't exceed the depth limit, it's also unlikely to gain you anything.
Now if they taught you enough and you used it on just the right dive you might be able to get more bottom time on that dive, but in this situation I don't think that's a good idea. You probably aren't diving a Nitrox computer, and probably aren't going to learn and use the Nitrox tables in this situation, and I wouldn't attempt to use Nitrox to extend a single dive if I didn't know what I was doing myself, since you'd really have no idea of your status.