Are you today perfectly sinless, JonC?I agree that God punished the wicked at the Judgment. I wouldn't call that a "need" on the part of God, but I agree with you that at the Judgment the wicked will be punished.
One issue is that this is not unique to the Penal Substitution Theory of Atonement. Most (all that I know of) theories of Atonement hold that in that "court" (Judgment) the wicked will be punished.
The difference is that Penal Substitution holds that those in Christ also stand condemned EXCEPT that Christ was already punished in their place.
That is unbiblical because it ignores recreation (that "in that court" the Christian has already died to sin, was made a "new creation's, his old heart removed).
It ignores or minimalizes regeneration in terms of salvation. Men are recreated without condemnation (that are made new creatures in Christ, in whom there is no condemnation).
We don't stand before God as wicked men whose sins have already been punished. We stand before God as new creations in Christ.
If you are not then you will stand before God as a sinner at the Last Day. Your only hope is the blood of Christ shed for sinners on the cross. Or are you a better man than Paul? 'O wretched man that I am! Who will deliver me from this body of death? I thank God - through Jesus Christ our Lord!'