To be honest I think that the xp being split up to party memebers that did not engage is not as big of an issue as the SP earning rate between different classes.
As an archer duo'ing with a Gladiator who is set up to tank, it is sad how much more SP I earn than him. Even though it would hinder me... I think that SP should be summed up and split the same way as experience.
A healer class and tank class can't rank up anywhere near as fast as a DD class if they are duo'ing together.