If the deification of man was the only example of heretical doctrine among Mormons, you might have a point. A very tenuous point since the Bible is very vague on the issue(we know in morality and righteousness, but anything beyond that we can only guess), but a point nonetheless.
It's not the only heretical doctrine, nor the one we should be most concerned with.
God was once a man -- Mormon doctrine teaches that as man is, God once was. It even goes further than this and claims that, at least when God was a man, He served a goddess whose name he keeps secret for sacred reasons. And that the cycle doesn't end there, but goes on into the past into perpetuity. The Bible, on the other hand, states very clearly that God just is, always has been, and always will be, and that God does not change. He was not once a man, but has always been God.
Denial of Christ's deity -- Like other Arian religions, Mormonism denies the deity of Christ. He is treated as an exalted spirit, but not God in the Biblical sense. This of course contradicts the first chapter of John, John 20:28, Matthew 14:33, I John 5:7, among other verses that make Christ's divinity undeniable.
Salvation by Works -- This is a quick and easy way to eliminate a particular group as Christian. Mormon doctrine teaches that it is works such as baptism, good deeds, missionary work, and following Mormon teachings that saves a person. The Bible, on the other hand, makes it clear that works do not cause Salvation, but rather faith. Other places in the Bible make it clear that the kind of faith that saves will produce works, but it is not the works themselves that save.
That many Mormons are unaware of certain doctrines is no excuse, and it does nothing to make Mormonism a valid Christian denomination, much less the One True Church that Joseph Smith was adamant about in his book "The Pearl of Great Price," calling all other churches corrupt and abominations before God.
Remember a few Christian doctrines that are very important:
1 - God is eternally existing in a place that is outside of space and time as we know it. They are, in fact, His creations. God does not change, and He has always been God.
2 - Jesus is God Himself, not merely an exalted spirit.
3 - Satan and Jesus were never spirit-brothers. Even before Satan's fall, Jesus was God and Satan was not, ergo Satan was his subordinate.
4 - Salvation is by grace through faith(a living faith that produces works), and is not by works.
5 - While they may add to or supplement previous revelations, new revelations from God cannot
contradict previous revelations. For an example of many conflicts between the Bible and Mormon doctrine --
http://www.irr.org/mit/bombible.html (It's not an exhaustive list, but it's a start)
It may also be noted that I don't care what early church fathers said. It's obvious from actually
reading the Bible that they were speaking on matters that the Bible is quiet about. In other words, they are not authoritative. The Bible is the final authority, and I refuse to hold beliefs that are not supported by it regardless of what some of the early church fathers thought.