Goodman Grey is a single unaffiliated individual, his Nemfection would not advance Nemesis cause one jot.
Goodman Grey is a single individual operating at a level where he interacts with Odin, and with Nicodemus Archleone.
Goodman Grey is one of about half a dozen beings with similar qualifications. You know what else there's similar rarity of? Faerie Queens. Senior members of the White Council. You know what's more-common? Fallen angels inhabiting Blackened Denarii.
Goodman Grey is
so good at what he does (undercover ops & infiltration) that he flies entirely under the wardens' radar (Harry recognizes Hannah from Warden reports, but nothing about Grey).
Goodman Grey has
repeatedly operated with Harry Dresden -- or in his close proximity -- in order to pay Rent.
Tell me again how Grey is of no use to Nemesis...?