I prefer the term "special". Shirou is more like an autistic person than someone who is an idiot or a fool. He has his own sense of rules and lives within those, because its the only way he knows how to live. He simply looks at the world differently than everyone else. And that's why the people around him love him so much. Its something that could only work with a character delicately crafted like his, unlike countless lesser written characters i could name.
I know it sounds like i am making weird excuses while singing praises, but that's just the sense i get when i go through the VN. A character that is intentionally and delicately written in a specific way for a specific purpose that matters to the rest of the story in a thematically important sense. A clear deconstruction of your average shounen hero who usually have no such depth to their actions while protecting their beliefs of the things around them, or at the very least is never explored to such a degree.