Opeth uses DADFAD for most of the Ghost Reveries songs. They use Drop D on Blackwater Park and Demon of the Fall. I can't remember what else they detune on, but why would a band with such diverse and masterful musicians tune the same way on every song?
I've tuned down for years, mainly because my favorite bands did, and I wanted to play along. At one time, I had 3 guitars with different tunings: E for Maiden, D for Bodom, and C for Inflames.
As I think people have already stated, something is not heavier just because it is tuned down, but I do think it adds a different level of feeling to the music.
Imagine this song in standard tuning:
If you listened to the first minute or so, and don't think it matters - i'd call you crazy - but skip to 3:50 and set up the change that happens at 4:12... I don't think that can be done any other way. I don't care if they're the 30,000,000th band to de-tune, that drop in that situation is fucking awesome.
I also think in this song, that low C# would just not be the same if it were an F - and this is by no means a "heavy song" but it wouldn't be the same in normal tuning.
I played with a rock band in college, and kept my guitar tuned to D for the extra range - nothing we played was considered heavy, but having those lower notes available and a whole separate set of open harmonics was great.
I think its kindof pointless to generalize about drop tuning, as there are many reasons to use it. It doesn't have to be a crutch - in fact you could look at it as an improvement on a classical instrument that has not changed in 100+ years. I think it is better to judge bands on a case by case basis, rather than trying to group them by how they tune.