The R Pi would be useless for what I do with an Arduino which involves a degree of timing, the Arduino can happily time things to a single millisecond (a single microsecond if you push it) and it has degree of accuracy (even the Uno with a ceramic oscillator), the R Pi having a powerful multitasking operating system couldn't be depended on at all, if some other stuff it was doing (like system maintainance) was guzzling the clock cycles what you thought should happen in 10 milliseconds might take 10 seconds. The puny 8 bit processor in the arduino only does the task in hand and it doesn't have a system to maintain.