Depending on how long the lines are, wrapping may turn out to be a pain in the neck because (part of) one or two lines take up the entire pane. So, though wrapping long lines works sometimes, it genuinely is impractical at other times. Also, like the other poster says, it's a matter of preference too. If someone likes to edit a line while having an eye on the lines below and above it, wrapping doesn't help.
I've always avoided this by formatting source code to fit in 80 chars, using "long" data format where possible, and just not editing big data files interactively (I use sed or awk).