Reading an article on a Mormon's page shows how a misunderstanding of Trinity, whether it be deliberate or not, can lead someone to boldly but false proclaim she knows the truth.
Here Jenny Evans seems to hint the concept of Trinity is just another word for Modalism. And attribute some major principles of the doctrine of Trinity to Mormonism's godhead principle instead. I've heard of plagiarism, but accusing the original author of a different doctrine after plagiarizing the original work from the author and calling it their own, that is just as perverse as it can be. As much as they are willing to reject Modalism, the "Latter day saints" still have difficulty understanding the meaning of "God" and Divinity.
The Christian cults couldn't understand the concept of trinity because they refuse to believe that the Godhead exists not in the boundaries of time and space. Therefore, they could never explain the three persons without having to deal with time (Mormon: Jesus exists after the Father) and space (Mormon: God has physical body).
What's the harm in this system, one might ask. First, it is a false testimony against the Truth. Therefore, undermining the full sovereignty of God, resulting in a polytheistic faith as well as devaluing the Godhood of Christ.