Why are enantiomeric excess % and specific rotation considered?
Enantiomeric excess is very important concept in organic chemistry because it clearly states the purity of one isomer over the other. For example, have a look at the structure of Thalidomide which is racemic (50:50 —R:S isomers) in nature. Between the two form, one is having very important medical property where as another form is sedative. In these cases, synthesizing enantiomerically pure (>99%) form is mandatory and in January 1996, the FDA announced it would consider further incentives for developing single isomer drugs, owing to their better pharmacokinetics prosperity, safety, and tolerability.
http://en.wikipedia.org/wiki/Thalidomide

These are the reason which makes chemists to target enantimerically pure form using latest available technologies like, Chiral GC or HPLC. 

Specific optical rotation is very old concept to judge the enatiopurity of the compounds but its not accurate because it depends on various factors like concentration of the sample, solvent which we use, temperature, etc. Even small impurity in the sample gives drastic change in the rotation value. Thats why chemists adapt latest technique like HPLC of GC, nevertheless, chemists still carry out specific optical rotation to make sure that the enantiomerically excess compound has some rotation.