Sccoby set up not ideal for heel 'n toeing, big gap between pedals. However, I tend to do this automatically now, not just to go quickly. In fact I ***** it up more trying to go quickly than not thinking about it every day.
Advantages: less strain on clutch, gearbox, drivetrain, in fact just abouyt everything except brakes.
Disadvantages: None that I can think of, except thinking you are really good at it all the time and fluffing it when you really need to get it right!!!
To be fair, advantages in a scooby are not that great I don't think for going quickly, because the car is so capable anyway.
If you really want to see why people heel 'n toe when going quicky, try hiring a Caterham or equivalent for a day on track, brake really hard for that hairpin whilst grabbing 2nd gear!! You will soon know why.
Basically the back wheels lock up because you are almost at that point with the brakes alone and it is similar to trying a handbrake turn into a hairpin at 40 / 50 mph... Not easy to keep on track.
At the ragged edge in a Subaru, I imagine there would be performance advantages in keeping the car smooth....
p.s. I am no expert, so I could stand to be corrected.....

 

I found this most useful on track, despite what some have posted it is quite easy to do (see above)and makes for smoother deceleration. I have fitted some of the cheap cast aluminium Sparco pedal extensions, these helped a lot. The 2nd gear syncro on my car isn't too good so it helps to make the change faster.
Don't really bother on the road unless 'pushing on' a bit.