On the RPi, nobody cares what OS is run, as long as it does those embedded bits, like control your pinball and shit.
With the first Raspberry, this was definitely true, because the Pi was too sluggish to use as a regular desktop. The Pi2 is significantly improved in this area. I use mine as an internet browsing box quite a bit. But, as you say, the current mix is probably 80 or 90 percent embedded usage. That’s changing though - and many of these types of machines are being used for media (kodi media boxes) - especially with the higher powered SoC/SBC boards like the Odroid.
They’re being used for NAS storage (esp. Odroid, Cubie) and for specialty server systems (distributed control, home automation, internet client, media center servers, weather monitoring, data logging, etc). So it’s much more than just dancing robots. They’re selling millions of them.
The desktop is changing quickly, and the traditional form factor is getting close to EOL. Microsoft is admitting that the hayday is over, as they try to make it in mobile, etc. Intel realizes that the chips are down (literally) and they’re trying to build traditional Intel architecture machines with lower power and cost figures, and are now producing very small form factor SBC boards that approximate the SFF of the Odroids and Raspberries. The pricing on those is still high though, relatively speaking. I use my little SoC/SBC ARM boards in several homemade setups using tiny enclosures (4x5x1 inches in one example) - and in total I probably use them slightly more often than the big old PC, which I still have.
Here is a little article that shows one of the boxes I’m using:
Intel won’t go away, so eventually we’ll see the “PC” in an entirely different way. The “normal” machine will be much more like my Pi in a box, a tablet, or just a super cell phone with more power that what they have now. I’m sure many of them will be Intel powered, but the mix of architectures will not be anything like it has been for the past 20-30 years. That’s changing, the mix will change, and an eventual port of Haiku could make a lot of sense. But - not necessarily today, since the current architecture is still a WIP.