You will only get one chance to capture the decisive moment. Today could be the day you take that one photo you will remember for the rest of your life. Why should it be a crappy in-camera JPG. Shoot RAW and process in LightRoom or even just iPhoto. But at least you’ll always have the highest resolution and most accurate shot you could have had.
A prime lens has only one fixed focal length. You can’t zoom in to get closer. Prime lenses are often faster to focus, better optically and cheaper for the same performance. With a prime lens the only way to zoom is with your feet.
On the street if you are zooming then you are too far away. Get in amongst the action. A wide angle lens will fit more into the field of view and will force you to get closer to compose the shot you want. You’ll want to flirt with long lenses for portraits and capturing that intimate moment across the street. But in the end, if you are a real street photographer, you’ll always end up back shooting wide.