Since everyone is a sinner, I don't get your point.
Isn't your statement the same as saying => "It isn’t a valid argument to advocate for a person because “sometimes God uses people.”?
Most sinners are doing the work of the devil, not of God. Now if God tells me later he used one particular sinner to do His Good Work, then great, I will become an advocate for that sinner. Otherwise, one has no justification to promote some sinner because God — Just Might — select that sinner to further His ends. To do so is presumptive.