PC Gamer has word from id's Tim Willits on RAGE controls, stressing the game's PC support allows the player to carry more weapons than, say, Duke Nukem:

Rage doesn’t follow the trend for letting you only carry two weapons at once. Instead, you can carry all your weapons at all times, and they’re mapped to the traditional number keys. “We were kind of back and forth on whether we keep to four buttons that you map to your favourites,” explained Willits. “But on the PC you’ve got a key for every weapon. And we follow the Half-Life control paradigm, where you can press the button twice to select the weapon.”