THX used to have more of a place in the A/V world back when Dolby pro-Logic was the dominant (and only) format. Surround sound was more of an exclusive and specialized hobby that much fewer were involved in. At that time, it was complex and very complicated for the average user. Certain standards (ie. THX) were implemented in attempt to stabilize and regulate the industry, to give it some guidelines and direction. Back when pro-logic reigned supreme, I beleive THX did serve a purpose. However, with discrete channel decoding and highly cabable speakers out there, THX is not such the necessity it once was. At this point, it is at best another DSP format and/or marketing tool.
I beleive the new yamahas have THX decoding in them simply as an addition to a vast suite of options and parameters for the user. Basically they tried to jam pack it with as many bells and whistles as possible (marketing also). At these price points, the units all pretty much sound the same anyway. It is features, reliability, and image that set them apart.
best,