Andy, sorry this question went so long without an answer. Thanks to Martin for the initial response (and thus prodding me). Everything Martin says is correct. Just to add that another consideration was the attritional impact of requiring people (a) to register for another login (because we didn’t think we could intergrate MediaWiki within WordPress) and (b) to learn the wiki ropes if they don’t already know them. We thought only a small minority of people would make it through these hoops.
Personally, I’d love to explore the potential of semantic MediaWiki with a community along the lines of ocTEL. I bet Martin would too. But we are a bit geeky, and ocTEL wants to be welcoming to non-geeks. Maybe we can find a compromise that streams the geek element without seeming exclusive of alienating to the majority in future iterations.