Form for polymorphic association

Sorry for bad English. Why we have so restriction in ActiveRecord?

/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.9/test/cases/
nested_attributes_test.rb

def
test_should_raise_argument_error_if_trying_to_build_polymorphic_belongs_to
  assert_raise_with_message ArgumentError, "Cannot build association
looter. Are you trying to build a polymorphic one-to-one association?"
do
    Treasure.new(:name => 'pearl', :looter_attributes => {:catchphrase
=> "Arrr"})
  end
end