In short, the Golf Guru believes that headcovers were a remnant of the persimmon wood era that serves no functional purpose today. He argues that headcovers did indeed protect "soft-as-cheese persimmon clubheads and the 'whipping' that bound the hosel to the shaft" but is unnecessary for the modern metal-wood.
However, I think that his advice is a little half-baked for several reasons. For one, some of us like to keep our clubheads free of dings and paint chips. Not only is maintaining the clubhead's condition aesthetically pleasing, but it helps preserve the club's resale value.
But one of the best reasons to use a headcover is to protect those precious graphite shafts. I knew a guy who never used a headcover, whether it was on or off the course. He would just let his clubs clang around in his bag unprotected on the golf cart and also in the trunk of his SUV. Then one day he was hitting driver on the driving range and the head snapped off. I think the clubhead flew farther than the ball!
You see, not using a headcover leaves the shaft exposed to the leading edges of any neighboring irons. Like eager beavers chipping away at a tree, the countless mini-dings from these irons can weaken the shaft to the point where it may break. At least that's what the pro shop guy says. Whether it's true or not, why take the chance?
Lastly, a headcover can protect your club from the prying eyes of thieves. With clubs such as the TaylorMade R7 TP worth as much as some people's cars, a generic headcover is the best deterrent for the growing golf club theft problem.
As you can see, there are some good reasons to continue using headcovers. But please, for the love of mankind, don't use a tiger headcover unless your name happens to be "Tiger." Otherwise, it's rather poseur-ish.