Is the pouring method of beer really important?
The release of aromatics is most certainly part of the reason for producing a head when pouring beer. You're creating a buildup of bubbles that contain carbonated gas coming out of solution; that gas can carry aromatic particles created during the brewing process. The more there are, the more aroma is released.

The "cap" theory is pretty dubious, on the other hand. The head isn't solid, it's just composed of gas bubbles that can be pushed aside by other bubbles as they escape out of solution.

Another reason not mentioned is that pouring a head allows one to carry a full glass more easily without beer splashing out. The bubbles in the head keep the liquid level below the edge of the glass, and they also provide a cushioning effect as the glass is moved around. It's odd, but from working behind a bar I can tell you that glasses with a decent head are easier to transport without spilling.

The specific pouring method described works consistently with most any beer, thanks to a moderate amount of contact with nucleation sites, which give the carbonated gas dissolved in the liquid somewhere to "grab" and form bubbles. Pouring the beer down the side of the glass provides contact and some (but not too much) agitation; straightening the glass then prevents any accidental spills as the level rises. Again from time behind the bar, I can tell you that this takes a little bit of feel and practice to do smoothly, but it's by no means difficult. You'll have to pour a few beers to try it for yourself.