Double clutching IS essential in some cars, but it does serve a very real purpose in today's cars as well.
Now, this doesn't mean you need to double clutch going UP the gears as stated in FnF and in that damn RSX-S commercial. All that does is make lots of cool engine noise.
Down shifting is a different story. You save your synchros a great deal of wear by double clutching. This isn't the same as rev matching. Double clutching gets all of the gears in the transmission up to speed so that when you do rev match as you shift into gear, everything meshes perfectly. Otherwise, the synchro is responsible for spinning up your entire transmission (not just the gear you are going into).
This pic should help.
If you are in gear, with the clutch disengaged, all of the gears in the transmission are spinning at the speed of the wheels. This is why you can't get into reverse cleanly when rolling. With the clutch disengaged and not in gear, the gears are free to spin down. If you are out of gear with the clutch engaged, the gears will spin at the speed of the engine. This is where double clutching is great. Spin those gears up, get back into the right gear without using the synchro, and then match the engine to the tranny's speed.
Ahhh.....there is nothing like a perfectly executed double clutch downshift.
Oh yeah, and when you are good at it, it doesn't take but an instant longer...but you should be slowing down if you are down shifting anyway, so why does it matter? You can keep braking.