Dangerous, old boy!
I think we’re largely agreed on the fact that we should present the Bible as ‘reliable, trustworthy,’ rather than “100% perfect, without flaw, contradiction,” etc. I take on board your note of caution, however, about the dangers of challenging inerrancy. After all, a great number of the heroes of our faith throughout history believed in something like inerrancy, so that’s good enough for me. Moreover, Jesus himself and the Apostles held a literal understanding of many of the more controversial parts of the Jewish Scriptures. I think you’re right that we should steer clear of questioning the Bible’s authority and make the focus of our ongoing discussion: ‘the question of purpose…what do we (or perhaps should we) use the Bible for?’
Turning to that question now, here are several angles we could take:
1.) First, your point that ‘our understanding of God is limited.’ If we present to unbelievers the truth that ‘God’s ways are high above ours,’ and make the point that, as God’s creations, we need guidance, illumination, or (to use a bit of Christian jargon), ‘revelation,’ to understand our creator, then the Bible exists partly to help us in our search. It’s there to sharpen our understanding, challenge our preconceptions and show us Christ.
2.) The Bible exists to act as an objective arbitrator when Christians (or others) differ in their opinions and understandings. As an extension of point one, if we are naturally limited in our understanding of God, then we are going to draw different conclusions from time to time. Instead of relying only on our subjective perspectives and fighting, we should all submit to God’s objective truth as set forth in the Scriptures (easier said than done!).
3.) Christianity is essentially a witness testimony that something real, tangible and historical occurred in 1st century Palestine. The first apostles witnessed Jesus’ resurrection and handed it down through the generations to us. As such, the Bible exists to tell us about this (and other) historical events: it is a record of what God has done for us.
4.) The Bible reminds us about God’s grace and steers us away from religion, but showing us that flawed, messed-up people can still be used by God. Rather than a religious textbook, it is an account of the lives of men and women who have walked with God, warts and all.
I hope that helps as a starting point for answering your question!