It's not that 3D movies aren't worth watching, because some totally are (see: Gravity). It's that Hollywood loves to shove it down our throat as something new and fancy and some sort of advanced technology and trendy and bullcrap like that. It's not!
Most of the time, 3D movies are made just to squeeze more dollars out of us (see: any movie in 3D not named Gravity). Comic god Stan Lee explains why he hates 3D perfectly in his latest rant. [World of Heroes via Laughing Squid]