It does not make sense for people to suffer for something God already has forgiven.
The forgiveness and redemption applies to the eternal consequences of sin. It should be obvious that our sins have temporal consequences as well, and many of which cause our suffering in this life.
Take murder as an example. We can ask and be forgiven by God for committing the murder, but does that mean we no longer have to go to jail? No. And often, those in jail suffer through it. Some temporal consequences and punishment is not so apparent in this life. So, I see no reason why this punishment/suffering shouldn’t be conveyed in the next. It isn’t eternal punishment or suffering, but it is uncleanliness that needs to be purged before one can enter the Kingdom of Heaven.
The simplest explanation of purgatory I have heard is this:
In this life, we are sinners. Even if one is forgiven, he is likely to sin again, still making him a sinner. This is something most protestants would agree with as well.
We should also know, given the verse from Revelation in the OP, that nothing unclean/corrupt can enter heaven. If we are still liable to sin, are we not still corrupt? It is possible for someone to not have the inclination to sin, but I would say that for the majority this is not the case. So, those in heaven cannot be sinners.
So, if one is a sinner on earth, and not a sinner in heaven, then by what other means can one go from being a sinner to not being a sinner without some kind of purification?
This purification is by no means separate from the work of Christ, but would have to be one of the results of the work of Christ on the cross.
My 2 cents:
What other things are there that might keep one too attached to this world instead of God? Entertainment (Movies, TV, Music, Video Games)? the Internet? Are these things not also things of which our attachment should be purged?