Rails 3 is a web framework. PHP isn't. I am always puzzled by the apple to oranges comparison: Rails vs PHP.
I normally would agree. But I think this is a unique discussion because the OP is asking why he should use something other than PHP. One might use Ruby because of Rails. So it's not a comparison of PHP and Rails directly, just reinforcing the point that languages/platforms other than PHP (and it's frameworks) are just as useful for web work as PHP is.