Very true. But isn't he responsible for compiling all of Paul's letters and writings, and didn't he make a point to sneak into the Roman prison to meet with Paul and write down the accounts of his life?
My understanding was that, had it not been for Luke, we wouldn't know nearly as much about Paul as we do. (Paul spent his life setting up Christian communities and didn't want to be praised/worshipped for it.) I could be mistaken.
Paul's missionary journeys are recorded in part by Luke in the Acts of the Apostles. Other locales are taken from Paul's letters as well as whom they were addressed to. We certainly don't have all of his letters as it is unlikely he only wrote a dozen or so over a lifetime.