Why offer headphones that allow the sounds to travel into and out of the units for audiophile listening ?
Because open earphones generally sound best (Although, I believe Denon make an extremely good pair of cupped headphones). The only reason to use cupped earphones is if you don't want to disturb others or if you are in a noisy environment like a car, bus or airplane. You will find that you listen at much lower volumes if you can cut out background noise - this is why pros use HD 25's everywhere - from soundmen at clubs/concerts to sports casters reporting at a noisy sports event.
I use HD-25's on the electronic drum set. I use AKG 240's for critical listening to music. The AKG's, however, are 600 ohm and they don't sound their best unless you use a really good headphone amp like the DAC1 while the HD-25 pros are awesome with the crap amp in my drum kit or the analog out of a Mac computer. I'd still place open headphones (AKG) ahead of the HD-25's for overall sound quality though (provided you got the right headphone amp)