I think the main purpose of Young Earth Creationism is to validate the Premillennialist view of the Last Things. Most premillennialists believe that God will create a new earth after 7000 years (premillennialism makes heavy use of numerology and 7 is supposed to signify perfection or completion - hence, God will destroy the old earth after 7000 years). Premills believe that Christ will rule 1000 years before the end of the world, and that Christ will set up his kingdom any day now. Therefore, they believe the earth "must" be right around 6000 years old. Naturally, people who believe in premillennialism also tend to believe in YEC. So, YEC is really all about religion.
Most Christians throughout history, while expecting Christ to return soon, had no belief in an exact timeline. That is why Christians from many of the older traditions do not see the age of the earth as having anything important to do with their faith. And that is why other Christians see the young earth idea as being central to their faith. This is unfortunate as some Christians anxiously read the news, basing their salvation on their correct reading of the "signs of the times" rather than trusting in the finished work of Christ.
In an odd way, premillennialism shares much with the old Gnosticism - one can have "secret knowledge" about some very esoteric things and this knowledge is an important factor in one's salvation.
I agree that this makes it hard to be a Christian witness. In American popular culture, many people are only familiar with the cheap grace of liberal Protestantism or the legalism of Fundamentalism (often including premillennialism). If people see you reading a Bible, they assume you must believe that Moses walked with the dinosaurs and somehow your life is centered on this belief! No wonder that many are turned off by Christianity if all they see is liberalism on one hand (if nothing matters, why bother?) and legalism over obscure, minor things on the other hand.
My own belief is that the age of the earth (and of it's preparation) is of consequence only in so far as it involves what the Bible says about God's relationship with man and what creation can tell us about God.
I think therein is the difference between Gnosticism, having secret knowledge not available to others, and the attitude of the Gospels, which was to tell everyone what was unknown in prior times but now is evident.
Obviously time and dates and how they are calculated is of great importance in the Bible as it looks both backward and forward, what has God said or done in the past and when, that would inform us as to what He will do in the future
And time periods are marked off as the Bible concerns its self with human history in the stream of time so that we can find our place in that stream. Christians have always, like everyone, asked what is in their future and sought a means of discovery.
For those reasons I think looking at cosmological models is important if we're going to use them to enhance our understanding of creation and Creator, not just debate over what we can never really know with certainty.