Refusing to contribute resources because a seat isn't "winnable" only strengthens the gerrymandering effect spoken of earlier.
But that's not the point. Gerrymandering is under control of state legislatures -- not contributors to viable House candidates.
One does not "refuse" to contribute money to an unwinnable seat. One sends one's scarce resources to winnable districts. If a winnable seat gets won by the opposition, the power of incumbancy could make it unwinnable in the future. This must not be allowed to happen.