It could hurt traffic, not directly - but indirectly. PR flowing through outbound links might eventually hit a site that's linking out to you, giving that link more weight in the eyes of Google.
Conversely, not linking out will mean that site gets less PR, and if the site in question links to competitors often then nofollow may be beneficial.
I wouldn't spend too much time on this, it's not really a thing that will make or break ratings. Going back and changing a bunch of links to nofollow seems overkill. My .02 cents.