Why put the bits of a port on non-adjacent pins?
Often this has alot to do with lead inductance. You need to keep your grounds spread out, and based on layout decisions in the chip internal to the package it may need to spread out the I/O pins. 

Would you rather have ground problems or spread out pins. In general, I use #define to handle which pin I am using and all but forget which actual pin it is. Since most people do this also, chip manufacturers know this and focus on electrical parameters.