I use polymorphic routes for readability purposes. This is useful to make the lines of code a bit shorter like this:
<%= button_to "Delete", [quote, line_item_date] %>
<%= button_to "Delete", quote_line_item_date_path(quote, line_item_date) %>
It is also possible to use them in controllers. For example, the two following lines for code are equivalent:
redirect_to @quote
redirect_to quote_path(@quote)