double-clutching is still necessary for racing applications, or just plain exuberant driving. This is especially true if your transmission has widely-spaced gear ratios and you are downshifting from a gear in high rpm to a lower gear, after sufficiently slowing the car down so the engine would not exceed redline upon engagement with the lower gear (slowing the car down while double-clutching requires heel-and-toeing).
even if your transmission has closely-spaced gear ratios, you will need to double-clutch if you downshift by 2 gears, e.g., in a hairpin turn.
if you rely solely on your synchros, without double-clutching and rev-matching, you will induce engine braking, which can upset your car's attitude in a high-speed corner.
i've wondered for some time what the tickmarks on the speedometer indicating the maximum speeds for gears 1 through 3 were for (e.g., in 3rd gear i can run 160kph at redline), considering there is also a tachometer. I've realized that these are useful when you are heel-and-toe downshifting at high speeds: e.g., from 5th or 4th gear at 180kph, brake until the speedo needle is below the tickmark for max speed at 3rd gear (160kph), at which point it is safe to engage 3rd gear, but of course after blipping the throttle for the tranny and the engine to match revs.
as the vehicle speed increases, the differences in rpm between 2 adjacent gears increases linearly. This means that at higher vehicle speeds there is a greater need to synchronize the gears before they engage.
i have a 5-speed 190e2.6 and from time to time practice my heel-and-toe double-clutching, which is useful considering the wide gear ratio spacing of the stock transmission. It is also quite satisfying when done smoothly.