How many people actually carry a spare clincher, or need to?
When foldable clinchers first came out - there were a lot of posers who wanted that "folded tubular under the saddle" look. Never fooled no one; ridicule was dispensed.
These days not so much, but people who ride with a spare clincher are usually on really long tours (backups of everything) or they know their mounted tire is about shot but can't bring themselves to throw it away until it's ruined.