One theory is that KyoAni was trolling the fans by making the same episode 8 times. They figured people would be dumb enough to buy it anyway. And in a way, they wanted us to have a feeling of Kyon and the SOS Brigade going through the same 2 weeks 15,000 times in a row as well.
Another theory, probably more true in this case, is that originally they were going to put the Disappearance Arc into the second season, but then decided to make it into a film instead, which "sort of" redeems them since it was really good.
As a result, they needed something to fill up a few episodes in Season 2, and for whatever reason they didn't want to use later stuff from the novels (or cut stuff from the earlier ones), hence why we got Endless Eight.
Now I wish they'd do a Season 3...
Alright, the second theory does make more sense really. Hopefully they'll make a third season eventually if there are more Haruhi light novels. :)