The story of the woman caught in adultery most certainly wields intrinsic value because it firmly sets in stone the superiority of our Lord Jesus Christ over the Law and portrays the cross as being the fountain of mercy that washes away iniquity. It strongly emphasizes grace over law; salvation rather than condemnation. It reveals to us the indispensable role that Christ played in rescuing us from upcoming judgement according to the Father's perfect standard of conduct. The text of John 7:53-8:11 strongly conveys to us the message that the Lord has been compassionate to us. Thus, it is no wonder why Christians would want to preserve this tradition in writing. Furthermore, there exists no legitimate reason to deem this event of Christ pardoning a women who was about to be stoned for harlotry as being false.
The oldest manuscripts available do not incorporate the text in question into John's gospel narrative. In fact, some manuscripts locate the passage of John 7:53-8:11 after John 21:24. There is one manuscript that has the passage inserted after John 7:36. Others have the story of the woman caught in adultery placed after Luke 21:38, or at the very end of Luke's gospel. There are many non-Johannine features present in the vocabulary and sentence structure of this passage. The motifs here are much more Lukan in nature (view source). It reflects characteristics of Luke's authorship. So John 7:53-8:11 would most probably read more naturally if placed at the end of Luke 21. There is nothing necessarily wrong with embracing the text as canonical Scripture, for it contains no doctrinal error and does accurately reflect the nature of Jesus Christ. It does not contradict any historical facts. Translators have simply reported their findings, namely that this text is missing from the oldest and most reliable New Testament manuscripts and is not found in the earliest translations. It simply was a scribal interpolation into the Gospel According to John.