I don't have a bluray player nor do I have a flat-screen TV,
but I am considering to buy the anthology/3D combo because these movies are of somewhat importance to me and the day will come probably sooner then later when we will all have a 3D-TV standing in our living rooms.
And I don't wanna buy it all over again then.
So maybe I will not able be able to watch it in 3D in the next few years but on that day I that get a 3D screen I'll be happy to already have Prometheus in 3D standing on my shelve.
That string of thought does not go for every movie I am going to buy,
I am happy with cheap secondhand DVDs from amazon marketplace,
but in case of Prometheus or the Alien movies it's different.