Then wouldn't that call into question the inerrancy of the Bible? I would argue not IF you take the position the Bible is inerrant in the message, morals, and spiritual truths it teaches. But some of the exact facts - well, those are much like the parable of Jesus, they are there to convey a bigger point.
Instead of saying, "Its' not in there so I don't need it." at least read the story behind why it wasn't included and see more than one view.
That's not what I am arguing, I'm saying what is in there is all you need to understand how to relate to God. It's not a historical, factual record of specific events, it's events used (sometimes figuratively, like the parables of Jesus - maybe other events as well?) to convey a bigger picture of man's continuing relationship with God, and how that relationship can be mended.
You don't use a saw to loosen a nut, and you don't use a crescent wrench to solder a circuit. The Bible's not meant to be a factual, literal history book IMHO - it's meant to be a relationship guide to teach us how to relate to God and to each other (the Golden rule).
Genesis 6 has been discussed for literal centuries with no agreement as to the Nephilim. We can speculate, but there is no resolution in Scripture. Some say they are from Seth, some say they are fallen angels. Some say angels can't have sex. Enoch explains it all and it pretty much makes sense. It also explains why God flooded the earth and why He told His kings to kill everyone, even women and children. These are hard facts to some beginning Christians looking for explanations.
Also, about the inerrancy of the Bible. The Bible is perfect, IMO, but some translations miss the mark. You have to go back to the original Hebrew and Greek to make sure you agree with the translators interpretation.
Just as an example, Gen 4:26. It says at the time of Enosh, men began to CALL upon the name of the Lord. Well, if you search other translations, you can find it interpreted that at that time men began to PROFANE the name of the Lord. Big difference. If you look at Jewish Study sources, you can see that most of them use "profane". Now is the Word inerrant or the translation a mistake? Another can be found in Amos 7:1. If you read most translations, you get almost nothing of the prophesy contained there. In the Septuagint, it names the king of locusts as Gog. Prov 30:27 tells us locusts have no king, so we know this is a prophesy of the end times and who the leader of the army will be. if you didn't search it out, the KJV or the NKJV would leave you hanging.
No one is saying you need to believe The Book of Enoch as part of your Salvation, but it does contain information written by someone that impressed the church fathers at the time of Christ enough to quote from it.