I'm not sure I'm adding much new stuff, but...
In the BONE, WSOs do very little actual navigating. Yes, it is still part of the Offensive Systems Officer's duties, but it's not the primary one, especially in a CAS environment. We're too busy talking to JTACs, punching coordinates into the weapons, running the radar and the targeting pod...well, you get the idea. And the defensive systems officer is just as involved...in an unopposed environment, he could be taking the other radio, backing the OSO on the coords being entered, plotting positions on the laptop...all this while the pilot flying is handling all the navigation. In an opposed environment (i.e., threat heavy), the DSO has his hands full running the defensive avionics, the chaff, flares, painting a picture for the rest of the crew, calling out threats to the rest of the package.
In short, WSOs are not human GPS interfaces to the pilot...we have a unique job. I don't know about F-15E's, but in the BONE, the pilots CANNOT complete the mission without us. We have the control over the bombs, the radar, the inertial navigation system...But equally, without the pilots, all I have is a $300 million static display. I've seen nothing beyond friendly pilot/WSO rivalry, and for every disparaging remark, I've probably heard 10 "I need a pilot/WSO to answer this question for me".
I'd also like to add to the list of O-6's with nav wings, including the OG/CC at Dyess when I went through the FTU, my current squadron commander, one of our O-5 ADOs, and the O-5 acting commander of our sister squadron. While a WSO may never be the "Aircraft Commander", they will very often by the "Mission Lead" (in charge of mission execution for their plane or formation) or even "Mission Commander" (in charge of the entire mission, including other platforms).