When you direct the highest-grossing film of all time, you're allowed to judge. No matter how silly or gratuitous the medium, that director has the last word because that director made the most money and injected some much-needed capital into our fledgling, little entertainment business.
While the rest of the internet may be up in arms because James Cameron, 3D-wizard and raging hypocrite, dissed Piranha 3D for "cheapen[ing]" 3D, it shouldn't take you by surprise. Cameron told Vanity Fair:
You’ve got to remember: I worked on Piranha 2 for a few days and got fired off of it; I don’t put it on my official filmography. So there’s no sort of fond connection for me whatsoever. In fact, I would go even farther and say that… I tend almost never to throw other films under the bus, but that is exactly an example of what we should not be doing in 3-D. Because it just cheapens the medium and reminds you of the bad 3-D horror films from the 70s and 80s, like Friday the 13th 3-D. When movies got to the bottom of the barrel of their creativity and at the last gasp of their financial lifespan, they did a 3-D version to get the last few drops of blood out of the turnip. And that’s not what’s happening now with 3-D. It is a renaissance—right now the biggest and the best films are being made in 3-D. Martin Scorsese is making a film in 3-D. Disney’s biggest film of the year—Tron: Legacy—is coming out in 3-D. So it’s a whole new ballgame.
To be fair, he was fired off the film. Who knows what he'd be telling Vanity Fair if things went differently. But I agree that there's a lot of potential for 3D movies (and disagree that Avatar is that much different than any other 3D sinkhole). Scorcese's upcoming 3D children's book adaptation sounds great, and Pixar's 3D versions of their films are only slightly less great than the normal 2D versions. Not exactly a glowing recommendation, but if a director can find a way to make 3D not weird and jarring to the viewer, 3D might see a real renaissance. Or maybe "weird and jarring" is 3D's actual future, in which case Piranha hit the mark.