RESTful Routes and Polymorphic Paths

I am a newbie. I will describe what I have observed. Perhaps some
kind soul will explain to me what is happening.

I need to pass a url parameter with a route. I find that this is
easily accomplished with a named nested route such as:

link_to admin_group_group_rpin_path(@group,
group_rpin, :as_of_date=>@group.as_of_date

However, when I try:

link_to [:admin,@group,group_rpin],:as_of_date=>@group.as_of_date

I find that the url parameter is not passed.

I know that I can pass a url parameter via

send("#{role}_group_group_rpin_path", @group,
group_rpin, :as_of_date=>@group.as_of_date)

to achieve the result of having the namespace be paramiterized and
also pass a url parameter, but it would be good to be able to do the
same thing with a polymorphic path.

Is this a bug or a feature?

Thanks, …