The middle distance sire has been on the wain for quite some time. I've often quoted the examples of Galileo and Montjeu (middle distance horses) compared to their stablemates Rock of Gibraltar and Giant's Causeway (milers).
On retiring to stud their fees were set as follows:
Montjeu (2001) – 30,000gns (Irish)
Giant’s Causeway – 100,000gns (Irish)
Galileo (2002) – £50,000 (Irish)
Rock of Gibraltar (2003) €90,000
As you can see, significantly more value is placed on milers than the true middle-distance horse.
It is still the case that sprinters and milers are more sought after at stud, mainly because they are more likely to sire fast, precocious two year olds (and get good early results) than a late maturing type of horse.
It is why so many St Leger winners, Ascot Gold Cup winners, etc are marketed as National Hunt sires, because no one wants stamina. It seems to be a case that people would rather breed for speed and get stamina through training or class.
Coolmore were obviously afraid that YEATS would be seen as nothing more than a National Hunt sire, so they did their very best to counteract that “stigma” by (a) setting his fee higher than most National Hunt sires at €10,000 and (b) placing heavy emphasis on the two year old winners in his family in his stud adverts. His 2010 stud card references his two year old ability and the Group winning two year olds in his immediate pedigree no less than 4 times!