[Terminology note: in this answer I'm using the word "clone" as the original poster used it; most of the devices he and I are discussing would be called "derivatives" in the Arduino taxonomy.]
Yes, clones are absolutely on topic. "Arduino" most often refers not to specific hardware devices but the ecosystem that allows people easily to play with a lot of different hardware and software, usually (but not always) using the Arduino IDE. Much as "PCs" quickly evolved to a general set of platforms with a lot of characteristics in common rather than a specific set of machines built by IBM, Arduino from the beginning, with opening of the specifications and designs, been aimed at this; the official introduction states:
All Arduino boards are completely open-source, empowering users to build them independently and eventually adapt them to their particular needs.
The commitment of both Arduino (the company) and the semi-official (SparkFun, AdaFruit) and unofficial community to this is demonstrated by:
I personally think one of the most beautiful projects I've even seen in the the Ardunio SE is Nick Gammon's torch locator, which involves nothing at all found on
arduino.cc except the IDE, yet shows so much of the spirit of what Arduino is: useful real-world interaction, a good smidgen of hobbyist-level EE knowledge (the analysis of the expected lifetime), and "we don't need no ARM CPU running Linux to do this." (The original question also hits many of these points, too, particularly with the novel application.)
Arduino has always been about experimentation and playing with not just the official things supplied by Arduino itself but with almost anything anybody can make that can be fit into the ecosystem. "Clones," made for many different reasons, and ranging from almost exact copies to very different things, are clearly a part of that.