What is your opinion about the Christian belief that some of the souls of sinners are placed in purgatory for cleansing purposes before they can go to heaven?

There's no scriptural basis for such a belief. The Bible describes that when people die, they simply return "to dust" a.k.a. break down to the bare elements they were created from and cease to exist. (Ecclesiastes 3:19-20) During this break down process, the person feels/knows nothing, completely unconscious. (Ecclesiastes 9:5, 6 & 10)

The only exception to this is when a person goes to Heaven. There's only a specific number of people going to Heaven-144,000. These are all chosen by Jehovah God and Jesus. (Revelation 14:1-4)

(New World Translation of the Holy Scriptures)