Borrowed from Justin D. Barnard, Purgatory and the Dilemma of Sanctification, Faith and Philosophy, 311-328:
The Paradox: According to Protestant Theology, a sufficient condition for entering eternal life is saving faith. However, a saving faith is not identical to a morally perfected nature because a morally perfect nature comes about through the process of sanctification. While a saving faith is a sufficient condition for eventually entering into eternal fellowship with God, it is not by itself sufficient for the possessions of a fully sanctified nature. When you add to this the orthodox position that the inhabitants of heaven are incapable of sin, and that the almost everybody, if anybody, never completely finishes the sanctification process before dying on Earth, a dilemma arises. Since the majority of people do not achieve moral perfection before dying, it would seem that either heaven is full of people who are still going to sin, or that saving faith is not a sufficient condition for eternal union with God.
The Paradox: According to Protestant Theology, a sufficient condition for entering eternal life is saving faith. However, a saving faith is not identical to a morally perfected nature because a morally perfect nature comes about through the process of sanctification. While a saving faith is a sufficient condition for eventually entering into eternal fellowship with God, it is not by itself sufficient for the possessions of a fully sanctified nature. When you add to this the orthodox position that the inhabitants of heaven are incapable of sin, and that the almost everybody, if anybody, never completely finishes the sanctification process before dying on Earth, a dilemma arises. Since the majority of people do not achieve moral perfection before dying, it would seem that either heaven is full of people who are still going to sin, or that saving faith is not a sufficient condition for eternal union with God.
ONE ATTEMPTED SOLUTION / PROVISIONISM: Some maintain that upon death God fills the gap between the lapsable nature with which persons possessing saving faith have upon death by supernaturally imposing the sanctified nature necessary to attain or occupy heaven. This resolution of the paradox is unattractive for the following reasons:
1) There are reasons related to the continuity of personal identity for thinking that God could not unilaterally bring about the kind of transmutation to which provisionism is committed.
2) Even if we assume God could do this, there seems to be no reason why God could not do it now. Thus, the fact that God could unilaterally bring about such a transmutation at death, but doesn't, makes God appear to be morally culpable for the evil that is the result of those who possess saving faith but are lapsable.
THE SANCTIFICATION MODEL OF PURGATORY SOLUTION:
Whereas the Catholic model of purgatory is backward looking and pertains to the payment of Earthly sins, the Sanctification Model of Purgatory affirms the sufficiency of Christ's work as a satisfaction for sin. However, Purgatory is still necessary on this model for those that need to complete the process of sanctification of its own momentum instead of God unilaterally transforming our natures so that we are incapable of sinning. In effect, the coming to completion of the process of sanctification after one's natural death is purgatory. This model of purgatory is forward looking, and has as its purpose the purging of the disposition to sin. Upon completion, the saved enter into heaven as denizens who have a settled disposition of virtue that is incapable of sin. This model preserves personal identity, and doesn't make God morally culpable for the actions that result from those who are still lapsable. Most importantly, this model of purgatory doesn't undermine the sufficiency of Christ's work, and the assurance of salvation for those who have saving faith.
1) There are reasons related to the continuity of personal identity for thinking that God could not unilaterally bring about the kind of transmutation to which provisionism is committed.
2) Even if we assume God could do this, there seems to be no reason why God could not do it now. Thus, the fact that God could unilaterally bring about such a transmutation at death, but doesn't, makes God appear to be morally culpable for the evil that is the result of those who possess saving faith but are lapsable.
THE SANCTIFICATION MODEL OF PURGATORY SOLUTION:
Whereas the Catholic model of purgatory is backward looking and pertains to the payment of Earthly sins, the Sanctification Model of Purgatory affirms the sufficiency of Christ's work as a satisfaction for sin. However, Purgatory is still necessary on this model for those that need to complete the process of sanctification of its own momentum instead of God unilaterally transforming our natures so that we are incapable of sinning. In effect, the coming to completion of the process of sanctification after one's natural death is purgatory. This model of purgatory is forward looking, and has as its purpose the purging of the disposition to sin. Upon completion, the saved enter into heaven as denizens who have a settled disposition of virtue that is incapable of sin. This model preserves personal identity, and doesn't make God morally culpable for the actions that result from those who are still lapsable. Most importantly, this model of purgatory doesn't undermine the sufficiency of Christ's work, and the assurance of salvation for those who have saving faith.

This book seems to argue for a similar point but at greater length of course. It is by Jerry Walls: http://www.amazon.com/Purgatory-The-Logic-Total-Transformation/dp/0199732299/ref=sr_1_1?ie=UTF8&qid=1364591826&sr=8-1&keywords=jerry+walls+purgatory