The only real reason to build up MIC's is for elites. When you train a levy pike phalanx from a lvl 5 MIC, it's still going to be just a levy unit so it won't be getting that much more training then if you train it from a lvl 1 MIC. Elite units already have the said training included in their meele defense stat, so there really isn't a need to give a boost from MICs.