The Catholics do not restrict purgatory to verses in the "Apocrypha". They use many New Testament verses to try to justify it as well. We Orthodox use the "Apocrypha" as well (although we don't use that term - Scripture is Scripture), although we do not believe in purgatory and do not believe that any verses in those books justify that belief, just as we do not believe that any of the New Testament verses justify that belief either. Nothing in the "Apocryphal" books teaches purgatory.
Exactly. Trying to pin purgatory on the deuterocanon is a cheep trick to get Protestants out of actually having to engage the argument.
Catholics use 2 Maccabees 12 as evidence for the practice of praying for the dead and making expiation for their sins. However, this doesn't necessary mean there is a realm called purgatory, but merely that commending the dead to God's care (through prayer) and hoping that sins they retained on earth will be loosed upon entering heaven (through prayer) is a proper funerary practice.
More often, 1 Corinthians 3:15 is used to show that there is a postmortum fire that purges sin, and I think we all agree on the canonicity of 1 Corinthians.
In any case, there is a simple logic to purging (though not necessarily "purgatory") that Protestants often miss. Catholic theology makes a distinction between the eternal and the temporal punishment for sin. Thus, if you commit a crime (say theft), Christ certainly died so that you would not go to hell for the theft, but that doesn't mean to don't go to jail in the meantime. The same is true of all sins- all sins merit damnation, but all sins also come with negative consequences in the temporal realm.
Purgatory is simply a way to deal with the temporal consequences. All sins are burnt away in that purging fire, not so that you can get to heaven on your own merits, but so that when you
do get to heaven you are free not only of eternal damnation but of all the penalties and consequences that go along with sin.
Once you accept the logic that A. there is a distinction between the eternal and temporal consequences of sin, and B. that Christ's death covers the former while only his merits (and the merits of the saints, and indulgences, and masses for the dead, etc.) cover the latter, then the need for some sort of purging because rather obvious.
I don't entirely buy into the Catholic interpretation of B. I think distinction A is necessary and that B is important for all systems of law enforcement (imagine if thieves, murderers, and rapists could just say "but I'm forgiven, officer!"), but I can't buy into the logic that Christ's merits, indulgence, masses for the dead, etc., are the way to push someone along through purgatory.
I'm not sure where I end up, then- Is there purgatory? Is there an instantaneous purging upon death? Is 2 Maccabees 12 a good model for a Christian funeral?- but the issue is far more complex than throwing competing biblical canons at one another.