Why don't you think it's useful?
I think that the critiques of it as a theory successfully demonstrate that its "Marxism" is hardly that. I.e. it use of the Marxist analysis is significantly flawed, even when it comes to definitional questions.

I think that proponents of "State Capitalism" theories of the USSR have a hard time accounting for commodity production and the accumulation of surplus value being so different than under capitalist systems. They also have a hard time accounting for the role of revolution and counter revolution.