Don Carson has written an excellent article examining why Penal substitution is under constant attack. The whole article is well worth reading.
In the light of recent debate that would displace Penal Substitution from the centre of our understanding of the atonement, this paragraph is particularly useful: I hope the paragraph becomes a chapter:
One recent work that loves to emphasize the Christus Victor “model”--Christ by his death is victor over sin and death--somewhat begrudgingly concedes that penal substitution is found in a few texts, not least Romans 8:3. But this work expends no effort to show how these two views of the atonement should be integrated. In other words, the work in question denigrates penal substitution as a sort of minor voice, puffs the preferred “model” of Christus Victor, and attempts no integration. But I think it can be shown (though it would take a very long chapter to do it) that if one begins with the centrality of penal substitution, which is, as we have seen, grounded on a deep understanding of how sin is an offense against God, it is very easy to see how all the other so-called “models” of the atonement are related to it. The way Christ triumphs over sin and death is by becoming a curse for us, by satisfying the just demands of his heavenly Father, thereby silencing the accuser, and rising in triumph in resurrection splendor because sin has done its worst and been defeated by the One who bore its penalty. Moreover, in the light of such immeasurable love,
there are inevitably exemplary moral commitments that Christ's followers must undertake. In other words, it is easy to show how various biblical emphases regarding the atonement cohere if one begins with penal substitution. It is very
difficult to establish the coherence if one begins anywhere else.