I like it rare, too. As you say, it's easy enough to cook it that way, but I like deli stuff quite thinly sliced, and I don't have an electric slicer, which is the only kind that would give me what I want.
So I ask the people at the deli counter to show me the beef so I can be sure it's rare enough. And then I ask them to show me one slice so I can be sure it's thin enough.