Or PhP vs Every Other Language Out There. The NoSQL crowd is mainly saying that RDBMS isn't for everyone, and that other storage methods need a bit of time in the limelight.

In each of your other examples, you are listing two things that are direct (or almost direct) competitors. It makes sense to compare them and then pick one over the other.

SQL vs RDBMS are further apart. There are cases where it makes sense to use both for different parts of your system. Sure you can force one to do the job of the other, but that shows you don't know when to pick one over the other (or were otherwised forced to do that...), not that they really target the same use-cases.