Funny you mention Jurassic Park III, I've been meaning to get the first two, but I will not buy the third one, even though Alan Grant returns, it's just not that good of a movie. Jaws is another example. I only have 1 and 2, don't plan on buying the others.
As for rules, I really only buy movies that have some decent replay value. There are plenty of other really good movies out there I've seen, but just will never buy. It's kind of a weird process of which ones I do end up buying, sometimes I will buy a movie I have never seen, but have heard awesome stuff about it. But I am trying to get a decent sci-fi movie collection going. But as of late, I haven't really wanted to buy any DVDs.
The double dips you mention, yeah, those can be brutal, but I got the Indiana Jones one out of the way, so any other would have to be some serious extra features included.
But speaking of double-dips (sorta) I really don't want to make the switch to Blu-Ray at all. I have a ps3, so I will definitely get the Indiana Jones Blu-Ray set (whenever they finally decide to release it) and maybe a few other favorites, but man, the price is just ridiculous. I saw the Mummy (the remake) for $30 on Blu-Ray, now that is a movie I will never buy regardless, but $30?!! HA!
Who knows how long DVDs will last, I'm hoping they keep making them, there is really nothing wrong with it, a DVD looks fine with an HD TV, if the movie is good, that's all that matters to me. I'm just ranting now.
If Blu-Ray does eventually take over (the Man would know...) do you think it will develop it's own, sorta phrase. Example:
VHS: "Hey, want to go get a video?" "Sure!"
DVD: "Hmm, lets go get a DVD" "Yeah!"
Blu-Ray: "Want to rent a Blu-Ray?" "Yes!"