User blog comment:Nick D Wolfwood/Questions questions questions!! Kurosakigate.../@comment-44061-20100624163959

Substitute Shinigami are rare and no doubt not that powerful or informed. Ichigo himself is a rare occurrence. Isshin would have to be a fully ranked Shinigami to have the power and knowledge he possesses. Also as you stated so far no one remotely recognizes the name. Substitute Shinigami that doesn't seem to be a viable or even ideal theory to use, unless one wishes to put a minor rarely used position in light to be important.