One virtue of birthright citizenship is simplicity. A child born in the United State is a citizen, and the government does not need to make any inquiries about either parent.
But let's assume, for argument's sake, that the Constitution does not provide for birthright citizenship in all cases. A host of questions would be raised that do not have simple answers.
First, let's start with a question under current law. The example that everyone seems to agree with is that the child of an ambassador is not a birthright citizen. The answer is a simple no, right? Not so fast. What happens if an ambassador has a child with a U.S. citizen? Would that child then be a citizen? In other words, does the "exception" to birthright citizenship only apply if both the father and mother were part of a diplomatic mission from abroad?