CAS exists only because the mechanical ASI does not know its own altitude and can therefore only be calibrated to read EAS correctly at one particular altitude. By convention this particular altitude is sea level. In theory you could calibrate a mechanical ASI to read EAS correctly at any chosen altitude, but such an instrument would not read EAS correctly if used at any other altitude for which it was not calibrated.
An EFIS however knows its altitude and can therefore display EAS correctly at all altitudes, just as it can display Mach number at all altitudes.
Aircraft such as the SR71 and space shuttle used displays of EAS (presumably) because they operated at very high altitudes at which the compressibility error of a conventional ASI would have been too great to tolerate.
The Eclipse light jet was also designed with a EAS display. You can read a position paper here setting out the arguments in favour.
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgELOS.nsf/0/46282217e067ba2c86257567006f6a5e/$FILE/ACE-05-32.pdf
Unfortunatly this article also has (at appendix B) a copy and paste from wikipedia which in this instance is rather misleading.
From an educational point of view EAS has the advantage that EAS, TAS and Mach number are linked by some simple equations which could be taught to student pilots and are easy to remember. By contrast the proper equation expressing CAS as a function of Mach number and static pressure is a horrendous affair with multiple nested exponents. It is not surprising that we don't teach the CAS equations to student pilots.
However as a consequence, many new first officers misunderstand the relationship between CAS and Mach number. It seems to be a common belief that the relation between CAS and Mach number depends on air temperature.
Last edited by Rivet gun; 28th Feb 2011 at 18:38.