If a DVD is released with a slipcover, I'll try and get it. Sometimes it's kinda cool (Heat S.E, Leon S.E, The Long Good Friday), sometimes it's pointless (Bubba Ho-Tep, Man On Fire S.E).
Usually, though, I buy it for aesthetic reasons; I'm not sure if any other Canadian schmoes have noticed this, but sometimes the cardboard slipcover only has the English text on the spine, which is much easier on the eyes than the English and French titles they try to squeeze onto the spine of Canadian DVDs (a pet peeve of mine that could warrant it's own thread).
Dunno...I'm kinda weird in the sense that I realize there isn't usually a point to them, but when faced with buying a DVD that has a loose disc and a slipcover, or a DVD with no slipcover and a held-in-place disc, I'll just put the slipcover on the good DVD and be on my way (which is what happened 2 weeks ago when I bought the Crimson Tide S.E; I'm sure I got some odd looks from staff and other customers).
As a side note, MovieWhore, I'm not sure if the Fifth Element slipcase is so much holographic as it is just "shiny." That's how my copy looks, anyway; feel free to correct me.