Mark Roberts gives an argument that hadn't occurred to me. Some people doubt the traditional authorship of the gospels. One thing that's strange about that view is that we have no explanation of why someone would choose the minor characters of John Mark and Luke, even if they did have some connection with Peter and Paul. Wouldn't it make more sense to choose someone who had actually met Jesus to serve as the invented author of gospels that are pretty much accounts of Jesus' life? If you're going to be inventing the authorship of the book we now call Mark, and you're going to say that the author who wrote it was Mark, who got his information from Peter, why not just say that it came from Peter? There was no Gospel of Peter at the time, so it wasn't as if the name was taken? Even if it made sense to choose a companion of someone who knew Jesus, it would be silly to choose a companion of someone who as far as we know didn't. That makes the choice of Luke extremely strange.
What Mark then goes on to argue is that this makes it far more likely than otherwise that the attributions to Matthew and John are accurate. Even if it seems really silly to question the tradition on Mark and Luke, it doesn't automatically follow that the tradition on Matthew and John is inaccurate. But it is the same tradition. These listings appear together generally, all around the same time, and we shouldn't expect it to be right on two of the four gospels but drastically wrong on the other two. That does increase the plausibility factor for Matthew and John a little.
Now I don't think much stands of falls on this issue. The only gospel of the four that makes any claim relevant to its authorship is John, and that's not exactly unambiguous (though I do think the most plausible expanation is that John is its author). But if we found out for sure that all four gospels were written by people we've never heard of, it wouldn't threaten conservative views on scripture's authority. It's just that this is a real difficulty for those who want to suspect that the tradition is unreliable. This is at least one reason for thinking of it as more reliable than many scholars, even some evangelicals, are willing to admit.