Paul was an extremely important figure in the early church as he was the biggest proponent of allowing gentiles (non-Jews) into the church. He was a very successful evangelist to that end. Your point was not lost on Paul's contemporaries, however, but Paul claimed his credibility from his famous vision of Christ on the road to Damascus.

Additionally, it is noteworthy that the authors of Matthew, Mark, Luke, and John probably never met Jesus either. Rather, they likely were writing a record of their oral tradition years after Jesus' death. This makes Paul's letters even more interesting, as most of them seem to have been written by Paul himself, and even though Paul didn't meet Jesus, he almost certainly spoke with people who had.

Finally, the reason anything would play a role in the bible depends on whether it is canonically useful. Paul's letters include quite a few important theological revelations. Note: give me until I get home to add a few sources.

Edit: Bart D. Ehrman's The New Testament and Stephen L. Harris' Understanding the Bible.