This is a BRD's bread and butter. This is the real reason tanks love BRDs.
Elegy (specifically Carnage Elegy) gives the mob an effect of Slow +50%. If the BRD has nothing else--no ballad, no minuet, no madrigal, no lullaby--this is the one song that BRD must have.
Horn +1 increases the effectiveness of Elegy songs. As such, every BRD worth his salt will macro in the Horn +1 when singing Elegy.
The chance of sticking any BRD debuff, such as Elegy (but also including Lullaby, Requiem, and Threnody) is tied to the BRD's Wind or String skills. The specific skill that is used in calculating resist rates is related to the instrument the BRD is using to sing a song. Since a BRD should always be singing Elegy with a Horn +1, a wind instrument, his resist rate will be determined by his wind skill.
Consequently, since Elegy is so important to stick, a BRD will want to attain the higest level skill in Wind Instruments as possible.
So why is it so much more expensive than the string earring? Because before level 60, ALL of the BRD's debuffs are tied to most effectively to wind instruments, not just Elegy. So this earring has multiple uses whereas the use of the string earring isn't nearly as apparent.
But, because of this perception, compouned by run-away inflation, this difference in use has become overstated, and the proportional difference in price between the wind and string earrings is rising exponentially.
The difference is just as stark between the wind and string torques. This is confusing because by the time a BRD can wear the torques,he will already be using a stringed instrument for Lullaby, and will soon be using a stringed instrument for Threnodies. But that's the economy for ya :/