That's a good point. However, when a function like 'string concatenates strings like 'join concatenates lists, it's most consistent for (string) to be "", right? You can't have (join) return "" as long as it returns nil, unless you also make nil and "" the same value.
Now, I'd trade bits of consistency for the sake of significant convenience, but I don't think there's significant convenience at stake here. I never use 'join or 'string for concatenation in a place I couldn't use + nil or + "", which are just as convenient. If we did that instead, we'd have a smaller vocabulary with less room for second-guessing the design. So IMO, 'join may well be useless, and the main value of 'string is that, when used with one argument, it plays well with ssyntax and 'map.
