They are the closest thing you can have to whitespace. That's the point. Actually come to think of it, they are underlined whitespace ;-)

As for a zigzag effect I think CamelCasing is at least that bad, maybe moreso.

Wondering if I get points for bringing early Medieval history into a discussion on coding conventions ;-)

What do you mean by zigzag effect?

The it isn't actually

word+empty-space+word

but

word+line-down-below+word

so your eyes go zig-zag from the taller glyphs of the word to the low glyph of the underscore (vertical zig zag motion), instead of jumping to the next world (horizontal).

funny, I was just checking because I have never noticed my eyes doing that and afaics my eyes still scan horizontally. I don't need to look down to see the underscore. I guess YMMV?