Think of the headphone can as a miniature speaker enclosure. The same rules apply as to speaker cabinets, just on a smaller scale.
The ideal speaker enclosure is one where the distance between the front- and rear-facing wavefronts is infinite, such that there can never be destructive interference between them. The natural resonant frequency of such an enclosure is irrelevant, and no acoustical tricks are necessary so there are no unnatural peaks or dips in the bass response.
You'd come closest to that ideal by mounting a speaker in a rigid wall with a huge open space behind it serving as an infinite baffle. Not very practical. Except with over-ear headphones, where the two wavefronts are effectively isolated from one another - but only if the back side is open to the world.
Of course, once you open up the back side you lose the major benefit of headphones, which is isolation. Now your headphone sound leaks out, making it unusable for tracking, and outside noise gets in, making it unusable while riding on a bus.
A semi-open back has vents to allow low frequencies to escape, as a compromise between fully sealed and fully open. Semi-open cans offer better isolation than open-back headphones and better low-frequency response than closed headphones. (Or conversely, worse isolation than closed and worse frequency response than open.)
[EDIT] BTW, Mike, I do use open-back headphones (Sennheiser HD-558) and they're wonderful. Not for tracking, obviously, nor for listening to music on airplanes, but rather for editing and as an alternate reference for evaluating mixes. And of course, late-night pleasure listening in the dark. For tracking, it's the HD-280Pro with its good isolation. And the trusty ATH-M50's for pretty much anything and everything else. Those are the ones I'll be packing for the beach when I head to the Philippines in two weeks.