How often do you need a spare tire riding around in the city? Foldables are useful for centuries or bike trips, but not for everyday commuting.
Wire beads keep the tire in round and make changing mindless; foldables have good stretch, but situating the tube/tire/rim takes more time than with a standard wire bead. The price difference is the biggest factor though.
Honestly that's what sold me. Continental Ultra-Sports on a Deep-V isn't really going to benefit from a folding bead. Even the employee at the bike store agreed. Saved me 9 bucks!