Wonderland.

Well, which one is it? Those are 2 very different reasons.

I ended up having to rebuy "The Devil's Advocate" due to it becoming unplayable (never figured out why that happened) and was glad that I was able to get the original recalled release.

Shield: season 1 because my lil sister f'd up one of the discs.

Special Edition:
Alien
The Big Lebowski
A Christmas Story
Clerks
DareDevil
Dazed And Confused
Donnie Darko
Empire Records
Ferris Bueller's Day Off
Halloween
Goldeneye
Goldfinger
Léon: The Professional
Mallrats
Monty Python And The Holy Grail
National Lampoon's Vacation
Office Space
Platoon
The Princess Bride
Reservoir Dogs
Stripes
Superman: The Movie
Superman II
Terminator 2: Judgement Day

"Unplayable" Copy
Donnie Darko (theatrical cut): friend scratched it
Jackass: The Movie: same friend scratched it (I loaned both movies out at once)

Dragonheart: Collectors Edition wouldn't play, so I replaced it last month with the HD-DVD version.

Frankenstein Created Woman-wouldn't play.
Never rebought just for an SE
Lost count of those I rebought to upgrade to 16x9

I've rebought Aliens, Alien 3, From Hell, and Army of Darkness.

The original SE of Aliens was actually the first DVD I every bought. Rebought later as part of the Alien Quadrilogy set. The original SE of Alien 3 I bought used, only because I thought it might go out-of-print (in light of the then forthcoming Quadrilogy) and it would be the only way to get the theatrical cut of the film. Again, rebought later as part of the Alien Quadrilogy set. The 2-disc director's limited edition of From Hell I bought used when it was out-of-print and new copies were going for $30-$40. Rebought recently for $6.99 brand new, since my used copy is pretty scratched up (it was the first used dvd I ever bought). And, finally, I bought the Army of Darkness Bootleg edition and then rebought it in the form of the Boomstick edition a couple of years later for the better transfer of the theatrical cut.

Unplayable-Casino (SE)

I don't think I've ever re-bought a DVD for either reason. Never had one go bad on me, and I've resisted the urge to double-dip for an SE type release.

What helps is I came late to DVD, in 2003. So many movies that had second and third releases, I didn't already own, so I started with the SE.

I do have a few original releases that have since had newer releases. Among them, Star Trek Nemesis, Fletch, Ferris Bueller, Trading Places... but the new editions haven't been enough to motivate me to buy them again.

For now, unless it's something I just gotta have, if there are older movies out that I like, when it's clear it's an old release, I will patiently wait for a new one with improved transfer/features. If a move is in that category, and it never comes, so be it. I must not want it bad enough.

Upgrades:

Dazed and Confused twice
Fast Times once
Breakfast Club once
Dawn of the Dead twice
Halloween twice
Office Space once
A Bridge Too Far once

Soon:
Wall Street
Red Dawn

Ditto. First copy started skipping and freezing in the last twenty minutes.

Unplayable - Bound

Sue

Special Editions:

Alien
Batman
Batman Returns
Batman Forever
Daredevil
Dawn of the Dead (1978)
Elektra
The Exorcist -- At one point I had 3 copies of this movie in my house. I bought the regular 25th anniversary, the version you've never seen then the big box 25th anniversary.
The Fly (1986)
The Fly II
Friday the 13th (Parts 1 - 8)
Halloween
The Little Mermaid
Mallrats
National Lampoon's Vacation
A Nightmare on Elm Street
Summer School
Superman
The Texas Chainsaw Massacre (1974)
The Toxic Avenger

Unplayable

The Exorcist -- Because my asshole friend scratched and refused to buy me a new one

And I'm sure more, that I can't think of right now