Little bit of interesting history on this was just a google away: http://blog.revolutionanalytics.com/2008/12/use-equals-or-ar...
http://stackoverflow.com/questions/1741820/assignment-operat...
The = operator is only a "top level" construct, while <- can be used anywhere. Now exactly what those limitations are may be a bit subtle (the OP's point), so I also just use <-.
Little bit of interesting history on this was just a google away: http://blog.revolutionanalytics.com/2008/12/use-equals-or-ar...