Here are some points to consider (also, this is not intended to be rude, just a brain dump with little thought on tact):
- The power requirements for small boards such as the Arduino line or RPi cost far less in energy than a typical PC.
-Resource requirements: Take BrewPi for an example, you do not need a PCs worth of resources to accomplish its tasks, so it's fairly wasteful to use a PC for a project like that.
- There is a fundamental difference between the purpose for an Arduino and a PC. An arduino is a microcontroller, its single purpose is to perform I/O. A PC is for general computation (spreadsheets, games, web browser, email, etc etc). For dependability and uptime when controlling physical devices, you go with a microcontroller (it doesn't have to be a dev board), all of your appliances do not use PCs to run.
-To extend my second point, an RPi is closer to a PC than an Arduino as it's a general computing device; we can't really compare the two. I wouldn't use an RPi to perform a job that an Arduino could do (i realize that you technically can).
-Keep in mind, as you said, something like Arduino/Spark/Baegalbone is ultimately a prototyping/development board. They are technically not meant for use in a production capacity (though honestly, i do). Technically after you have completed your project development you would complete it by building a custom board(s) for it.
-I don't have a pc, I have a laptop I use for development (my job), a tablet for pleasure, and a phone. I'm not using any of those things for single-purpose projects; that's exactly where my handful of arduinos and raspberry pis come in; and none of my projects use a screen or a nice case because they are all tucked away, busy automating my life and just sending me notifications when something goes wrong (rare).
-it's not in everyone's blood to find fun in programming, but i do so that's the big reason for me to use these boards. Otherwise i'd just buy a product that does the same thing for twice the price.
hope that helps give a different perspective.