The main driving force behind my designs is the hackability. The first thing I ask myself is "what could someone else do to this?", and if the answer is "not alot", then I am not happy

The decision to use the dsPIC as opposed to a codec was primarily that of hackability. I make the device and get it doing what I want, then throw it out there and let others run with it. So, it has to be something that others can reprogram, and what better than a dsPIC for that?
The other option of a codec + MCU would be kind of as hackable, but the MCU I would want to use then would bring the cost in above the dsPIC.
And you are quite right, you don't "need" the Arduino in there. The dsPIC is quite capable of running the audio and doing other things at the same time. In fact, the very first writing of the firmware didn't have the Arduino in place at all - it was just reading the SD card files in sequence and playing them.
There are a few IO ports available on the board (mainly linked to LEDs for a VU / Spectrograph), which I break out to a header as well, plus you could re-appropriate the analogue input and the Arduino interface pins to do whatever you wanted. The reason for the Arduino though is for people to be able to use the WavePro without having to know how to program a dsPIC. It functions as a good audio playback card (with record feature that is "usable"

) out of the box.