Um, since math has existed? Compilers double as automated code error checkers. Or maybe you like checking your code's syntax by hand?
Node.js is single-threaded: OK, were you planning on serving clients with a single server instance/process?
A single language client- and server-side offers little to no benefits: Yeah, you're right. Why would I want to a single test suite for my client- and server-side code? Why would I want to (securely) share model definitions between the client- and server-side? Why would I want to optimise one code-base instead of two? Why would I want to debug one language instead of two?
You finally make a good point. +1 for you.
MongoDB is for people who can't/won't learn SQL/the relational model: I'll admit that SQL is not my area of expertise, but my naive understanding is this: in SQL databases, you normalise your data by default until you hit performance issues. In NoSQL databases, you denormalise it by default. The decision on which one to use should depend entirely on the data you want to store.
Take it from me, you know NOTHING about data persistence no matter how much you think you do. Relational Algebra is the basic math governing manipulating, querying and storing data as tuples. It governs MongoDB (and other NoSQL tools) too, just that the authors of those systems basically lopped off all the hard parts of the relational algebra and implemented just the easy bits. And they did those parts quite poorly when compared to traditional RDBMSes.
You got a point there. However, it seems that lately this rule isn't understood by those that parrot it.