The biggest reason I have NOT upgraded is that TWOK was the only one in the set to have gotten a proper Blu-ray treatment, meaning it was scanned at high resolution from an original film. The rest are what I call "fake-ray" meaning they ran the file from the DVD through a sharpening software. Fans say they are sharper but there is obvious software involvement and not the usual Blu-ray quality. So I'm going to wait for a proper Blu-ray quality scan from film and director's cuts for the super-mega-signature-ultra editions, which we all know are inevitably coming. The 2-disc DVDs are fine until then.
This is pretty much my situation as well. I don't own any of the Trek movies on Blu-Ray, but the only one I have
been tempted to pick up is TWOK for that very reason. I have no doubt that the other movies all do still look spectacular on Blu-Ray, very likely much better than the DVDs... but they aren't real
HD upgrades, they've all been done on the cheap. And the latest individual releases of TMP, TSFS etc are the same transfers. I also have no doubt that 'true' HD rescans of the original film prints (ala TWOK) will be done for all
the movies at some point in the future, so I'm more than happy to save my cash until then.