In biking, it's better to be the belt and suspenders sort of rider. Especially going long distances.
(All of my British friends: quit snickering!)
For example, if you get a flat, it's usually easier and faster to just put a spare tube in, instead of trying to find the hole, patch the hole, wait for the glue to dry, etc. On the other hand, patch kits usually carry six patches in the space taken up by less than one tire. Furthermore, both patch kits and spare tubes age. Patch kit glue will fail, and spare tubes will rot. And there's a side problem that every now and again you find that the spare tire has a defective valve or something.
Point? Well, it's much less likely that you will be able to avoid getting stuck if you carry both a patch kit and a spare tube.
Also, it's always good for the larger cycling world. If I see a roadie stopped along the side of the road, I don't have a spare tube to offer them, but I do have a patch kit.
I'm also a believer that the world has a sense of humor. So, after I was talking to some other cyclists while stopped partway down a mountain, one of them mentioned how his frame pump failed on him the other day and he was very glad he'd brought a CO2 inflator as well. It turns out that enough water had gotten into his frame pump while he was riding in the rain to cause the gaskets to fail.
This instantly made me realize, because I rode in the rain all winter long with my frame pump on the side of my water bottle mount, that if I didn't get myself a CO2 inflator post-haste, I'd certainly get myself in deep trouble in the worst possible way.
This is doubly so, because mountain bike tires don't get flats nearly as often as road bike tires. Which means that I'm both out-of-practice at quickly fixing things and most likely to experience a flat in the midst of a long ride, not close to home.
So, yeah. I picked up a Genuine Innovations Ultraflate Plus as the backup. The big thing is that it offers logistical simplicity. If I was a weight weenie, I'd get the little CO2 inflator that just screws atop a threaded cartridge, but this way, I can use most any size of CO2 cartridge.
Actually, I ended up buying the Ultraflate and then I ended up replacing the main pump, because it wasn't pumping right anymore either...
I also had a "Don't Worry, I was a Boy Scout" moment yesterday. See, I took a nice 47 mile ride, starting at home, riding the Stevens Canyon trail, and then descending down Page Mill into Palo Alto, visiting my friend Elea, and then returning home. And, at the end of the Stevens Canyon trail, I was taking a break with two other mountain bikers. One of the two had a nice bloody red spot on his leg. So, I offered him some of the medical supplies that, just because I figure it's useful to have, I carry in my trunk bag.