Also, you'd probably do more with the Pi as a programmer beyond turning pins on/off -- for example, you could hook up a camera and run opencv, then write a small app to control a water gun that shoots squirrels off your bird feeding post.
When you start doing more with arduino, you typically want network connectivity, and most of the wifi/ethernet shields for the arduino are the cost of the entire RPi computer. I think the RPi does fill a good niche in the hardware world.
As far as programming, simple is easy on Arduino. The limits are further away with a Raspberry Pi. Say you want to log some data for later analysis. Trivial on a Pi. Anything more than a few kilobytes and you are wiring an SD card to your Arduino.
I'm currently using Arduinos for well defined, simple functions, but for more complicated functions. I will use Raspberry Pis where the BeagleBone CPU is overkill, but Arduino will always win for low power use.
http://wickeddevice.com/index.php?main_page=product_info&...
Many of your other points are valid. For tons of projects a Pi would be overly complicated overkill, but I can think of many times when I'd want more video in/out capabilities, plus the ability to blink some lights, drive some motors, and accept non-usb sensory input.
I'd like to know what kind of stuff you can do with the GPIO pins directly on the Pi, via /sys/class/gpio. Are they just digital (0,1) or they can read voltage levels too? (electronics newbie here)
[1] www.pjrc.com/teensy/
Here's a tutorial on hooking the raspberry pi with one of those chips to give you analog input: http://learn.adafruit.com/reading-a-analog-in-and-controllin...