The vast majority of the projects the RPi are being used for could be done by a microcontroller. So when you compare them against other devices used in the same application then for the same cost of an Arduino you get 15x the speed, 100x the RAM, and Ethernet, and OS with a complete TCP/IP stack ready to go.
There certainly are a lot of overpowered Pi projects out there. Though, the biggest benefit is a full Debian Linux OS running on the board. You can easily create a really nice web based interface and run it all from the board using WiFi or Ethernet without cobbling together a bunch of Arduino shields and figuring out how to communicate with them via serial. An HMI plus logic controller plus development environment wrapped up in one unit so to speak. You also don't need a separate PC to develop, just a keyboard, mouse, and monitor.
Anyone who really complains about the price of the RPi is expecting it to be something it's not. There are plenty of boards far more powerful than the RPi for under $100 and they don't sell anywhere near as well, don't have anywhere near the same number of projects being developed for them and don't have even a fraction of the community support.
No one complains about the Pi's price. In fact, it is its greatest selling point. I think my biggest complaint is that the Pi gets the most press and the others are drowned out by the sea of "Pi noise". There are a few other boards out there:
The Beaglebone Black which is another 20 bucks and has a much more powerful CPU, hardware ethernet and better GPIO. Its layout kinda sucks though, the single USB port is too close to the micro HDMI port which means USB connectors physically interfere with the HDMI port. And micro HDMI ports suck. Another problem is it was just announced that TI might not want to continue supplying the SoC for the BBB forcing the manufacturers to switch to a Broadcom SoC. So its future is unknown. Plus they insist on using Node.js as the primary engine for writing code. Dumb.
There is the UDOO. But it is pointless to cram both an Arduino Due (ARM based) and quad core i.MX6 on the same board. It adds a needless layer of complexity abstracting the I/O from the main CPU via a UART and secondary CPU whilst forcing the burden of communicating between the two on the user. A stupid setup.
After those boards there really isn't any decent competition that brings anything new to the table. It's just another i.MX6 or OMap board that doesn't offer anything that compelling. It runs Linux, big whoop. What about I/O? I need PWM, ADC, and GPIO. Not a few GPIO's broken out to a header. Bunny Huang attached an FPGA to an i.MX6 in his open source laptop. That was a brilliant move. But at $500 for the board I can but an ITX board with a PCI slot and pop a PCI FPGA card on it from Mesa Electronics for far less.
The Intel Galileo is another interesting board as it added arduino library and shield compatibility. So you have a board with Ethernet, USB, runs Linux and supports most of the Arduino libraries. So Arduino users can port their code and take advantage of on board ethernet, huge memories, threading and all the goodness that comes with a full blown Linux PC. That was a pretty damn smart move. But it still lacks CPU power, no display or GPU and its I/O is hung off SPI hardware instead of GPIO registers off an internal bus. So for every step forward, we take two backwards.
The Beagleboard-X15 brings a very powerful SoC to the table. My only concern is software support. If we can develop software for the PRU-ICSS as easy as an Arduino then we can really develop some serious applications. This would be a killer robot board. And the DSP should come with OpenCV support and easy to use libraries so we can write DSP code without needing to be a TI engineer or experienced embedded developer. Abstract the complexity using libraries and good documentation and you will cut through the Pi noise.