A five second bonus in blitz will typically get you to the end of the game "on the board" instead of "on the clock."
So you get some practice, especially in openings, without the sloppiness of Game in 5/0. And converting winning endgames (using just the 5 second bonus) is a great andrenaline rush.
Bullet, however, is a whole nother kettle of fish. There your just go blind and your nerves fray. At least for some of us older folks. 
I dont think there is anything wrong with playing blitz games, i think its a good way to make chess more interesting.