While we are permitted to have our own traditions (e.g. Romans 14), Scripture tells us quite plainly that it is sufficient on its own. It repeatedly commands us to neither add to, nor remove from it. In Deuteronomy 4:2, God even gives the Israelites this warning: "You shall not add to the word which I command you, nor take from it, that you may keep the commandments of the Lord your God which I command you."
That phrase "that you may keep the commandments..." gives us the purpose of preserving Scripture. When we take away from Scripture, we naturally end up losing that which the Lord has given us, but when we add to Scripture, the same thing happens. Our additions will always be inferior, and may actually result in failing to keep it.
Take Eve's addition to God's command as an example. When Satan asked about God's command regarding the trees, she said she had been commanded not to touch the fruit from the tree of knowledge. Yet, this command was not from God. She (or possibly Adam, having received the commandment in person and passed it on to Eve) had added it. Thus, when she touched the fruit, and did not die, she actually ended up with "good" reason to believe the rest of Satan's lie.
Sometimes, we "add" to Scripture to help us keep it. Verse divisions are an excellent example. God did not give us verse divisions, but they are excellent aids to memory, as well as reference. But when we start insisting upon our additions, we open ourselves up to further disobedience (and I say further because adding to Scripture is already disobedient), and maybe even apostasy.
Look at atheistic arguments. How many of them are based on Christian traditions rather than actual Scripture? For example, does scripture really say Cain and Abel were Adam and Eve's only children? This is an addition to Scripture, when in reality, Scripture mentions at least one other son of Adam (Seth) by name, and ambiguously declares that he had other sons and daughters (Genesis 5:4).
Altering Scripture in any way opens the door to further disobedience, and even apostasy. As Christians, it is vital for us to study Scripture, and not seek to change it. As I like to say, "the only pen you should use on your Bible is a highlighter".