I started writing a RoR medical office application. Then I thought what if I wanted every patient to have an RFID card and perform measurements (BP, Weight, etc.).
I found an RPi + Arduino project with peripherals, manuals and all I could ask. That I can handle, building everything from scratch (from software to hardware) would take a tremendous amount of time (due to lack of expertise on my part).
So as you said, even if you get me an x86 board at 50 USD I'd still go for Arduino.
I can recommend the Pi to someone who has never used Linux before because there are so many resources available that are Pi-specific. It's the same reason I recommend Ubuntu for newbies even though I prefer other distros for myself.
Not to mention Arduino seems to have shown commitment to their pinout configuration. I seriously doubt later iterations of this board will retain compatibility.
Pi isn't going anywhere, as they have a great understanding of the community they wish to create and do educational projects with.