A DNF because of a flat is just unacceptable for pros, but for normal trail guys they still suck. That’s why a lot of people turn to foam tire inserts, better known as mousses (French for “foam”) to completely eliminate the chance of getting a flat. The foam material is heavier than a tube and do wear down over time. The microscopic gas pockets that make up the foam get crushed every time you ride and after about 10 hours of hard riding, the mouse will most likely have shrunk to the point where it can no longer hold the tire on the bead of the rim. But having mousses in your tires allows you to ride much more aggressively in extreme and rocky conditions and some riders really prefer the “dead” feeling that they give the tire. Check out this video of Doug Schopinsky from Pirelli/Metzeler demonstrating how to install a mousse without using a tire machine.