I think it's just logic or deep pockets... the person who is going to pay $2000 or more for an R10 is probably going to considering high-end amplification. There just isn't much reason to hook a KSC-75 up to some uber Singlepower, for example, other than curiosity... the headphone's limitations are far lower than the amp's. Also, the Stereophile article I was skimming, New Media Metrics, indicates that a large reserve of power is required to meet most speakers' requirements for sound.
To quote: blah blah blah techie talk... "The power output per channel required to achieve this is 57W. To achieve 16dB above this?about the largest 2ms dynamic power requirement identified by my measurements?means the amplifier must be able to deliver 2.27kW, albeit only over this short time span, in order to prevent clipping." You can check out the rest on their website http://www.stereophile.com/features/404metrics/index.html.
That's a lot of power... just scale that idea down for headphones: although there are differences in impedance and sensitivity, the same demand for instantaneous power is there.