I did a fire by fresnel a few weeks ago. They’re so light and compact it’s silly to not carry one in a kit.
I think of these fresnel lenses as a way to extend your fire kit. In the right conditions, they'll save you a match or a scrape on your ferro rod. They don't take up any space in your pack or wallet, they weigh almost nothing, and they can't be used up. My experience has been that they work better with dark colored tinder.