I like your point. I've never liked dual citizenship, basically you've just put words to why. A person's loyalty should be to one country. Cruz at birth, I'd think, was "naturally" a Canadian citizen, probably eligible to be a citizen of the US or Cuba instead. Under that definition, Obama's question is not just where he was born, but that mom was a minor.
This is definitely a case where inept establishment Republicans put off something, I guess because they wanted to save Obama, and now in a situation where the dems won't return the favor.
The way I see it, Cruz is naturally a citizen of Canada (Canada sees it this way, and document such by issuing a Certificate of Renunciation, or whatever title they gave), and simultaneously, by operation of US statutory law, was made a citizen of the US. Cruz's US citizenship depends on the statute. Absent the statute, he would not be a US citizen.
Today, given that many if not most people find statutory citizenship and natural citizenship to be exactly the same, it is possible to be a NBC of more than one country. Cruz fits this pattern. In my view, if citizenship depends on a statute, then it is not natural citizenship. But most people are of a mind that if statutory citizenship attaches at birth, then the person is naturally born into that citizenship, because they do not go through a naturalization process.
Courts don't faithfully apply law or precedent. Courts are all about obtaining desired outcomes, so it's anybody's guess how a court would rule on the case, if it even took it (I think the court would punt to Congress).
But at least for academic purposes, it's interesting to read the history of citizenship law.