It adds to the decor. There are many DVDs in which the slip cover is, for the lack of a better word, shiny and nice to look at.
And there are some with a different cover. The Iron Man 2-disc one is clever in that there's a hole where his heart thing is and when you slip it off, there's just Tony Stark. FF: Rise of the Silver Surfer 2-disc's cover is interesting in that under the cover doesn't have the logo, just the Silver Surfer flying.
And if you guys are complaining about it taking too long to open a case with a slip cover, just throw it away. It's won't hurt you.