We need creeds to filter the cults from orthodox Christianity. If you take just the Nicene Creed, for instance, then no, it is not necessary to know the Nicene Creed in order to be a Christian. If you read only the Bible you will have all these doctrines. But if you deny any of the main tenets of the Nicene Creed, then you are seriously deviating from the Christian faith and there is danger of falling into heresy -- such as denying the full deity of Christ.
So the Nicene Creed becomes useful for the beliefs it denies just as much as for what it affirms.
Now some may say that we don't need the Creed in order to tell what a Christian is, we have the Bible. True! The Bible alone IS sufficient. However, cults, such as Mormons, Jehovah's Witnesses, The Way, and numerous Eastern Monists, accept the Bible as holy Scripture. They affirm that Jesus is the Son of God. What they deny is the succinct definition of what the "Son of God" actually means according to the Nicene Creed.
True, it is possible to use the Bible to achieve the same end. That is how the Creed was formulated -- by using the Bible. But the use of the Creed keeps us from having the same debates over and over again in each generation. We have known since the early centuries why Mormonism and The Way are cults -- even though these religions hadn't appeared yet. The arguments used against Arianism by Athanasius and the Church Fathers were repeated in the Nicene, Athanasian and Chalcedonian Creeds. These same points of belief refute most modern day cults today.
If you deny the Creeds, you deny Christ.
See more on the use of the creeds in "Christianity vs. the Cults" at:
http://www.forerunner.com/orthodoxy/X00 ... _vs._.html