What exactly is "common grace"? That is a question that is more confusing than the answers, for it seems that everyone has their own definition of what "common grace" is. McMahon puts forward a definition of "common grace" that is probably embraced by many. According to him, and summarizing Louis Berkhof's idea of "General Common Grace, [Louis Berkhof, Systematic Theology, 435. In Louis Berkhof, Systematic Theology: New Combined Edition (Grand Rapids, MI: Eerdmans, 1996)] "common grace" refers to "good gifts to the wicked" from the Holy Spirit to all men indiscriminately (pp. 112-3). McMahon did not mention this, but Berkhof did make it clear that the phrase itself (at least its initial iteration) is a late development from Abraham Kuyper and Kuyperians like Herman Bavinck.[Berkhof, 434] McMahon also gave another definition of "common grace" later in his work, which is probably held by some Neo-Kuyperians and is more heterodox, that "common grace" leads or even prepares men for special grace unto salvation (p. 460). This "natural light" definition of "common grace" is so obviously contrary to Scripture we can discount it altogether and deal with the more general version stated above.
McMahon rejects "common grace" while affirming God's indiscriminate providence to all. In other words, McMahon affirms the substance of God's goodness while denying this goodness the appellative "grace." McMahon's main point here is to state that grace is not in things but grace is always "being in Christ" (p. 128). If grace is always grace "in Christ," then certainly it would sound strange to state that the reprobate who are outside of Christ can partake of this grace. Thus, McMahon makes his case that God's indiscriminate providence does provide good to men but it is not grace.
What are we to say in response? To be sure, if grace is only found in Christ, then of course there is no such thing as "common grace." And we would certainly agree with the content of the goodness God has given to all men including His restraint of sin. But what exactly is grace? Of course we affirm the notion of saving grace, but can we say there is a grace that is not salvific?
Here, I think a look at the Noahic Covenant is in order. Under this covenant (Gen. 9:1-17), we see no promise of salvation from sin but merely the preservation of the creation order. [This is different from the covenant of Genesis 6:18-21 which is a salvation covenant from the Flood]. As it is in the context of a covenant, grace is certainly involved. While it can be argued that there is a salvific event that happened just prior to the making of this covenant, still the covenant parties include all living creatures (Gen. 9:12) and thus animals, which are not saved. Therefore, it seems that limiting "grace" to only being in Christ is too restrictive.
The phrase "common grace" therefore I think is a valid phrase and describes a valid doctrine, only insofar as it is limited to the blessings of the Noahic Covenant. That said, since the Noahic Covenant has regard to earthly realities only and is thus penultimate, the end result is the same as McMahon's case in denying any idea that God has any salvific intent or desire of any kind towards the reprobate.