Yes, it's underpowered and possibly overpriced in comparison to (x, y, z,...)
But the Raspberry Pi has a large and growing ecosystem behind it -- developers (hardware and software), users, and more.
The Arduino is a similar beast -- underpowered, overpriced, and with a tremendous ecosystem, approachable and available to new classes of users.
As an example, look at what Adafruit is doing with Arduinos and the Raspberry Pi -- making them available, accessible, and useable by a wide audience, not just those tho are comfortable rebuilding kernels.
Look at other historical examples -- the underpowered 6502 (Apple ][) or that atrocity with 640k is good enough for anybody, right?