I think the current behaviour is correct. It seems to me that :as changes the url and only the url. If it starts to have implications on the controller, that would be very confusing. Say you would use :as to provid l10n names for routes, this is something we do very often. So we have a route like this:
map.resource :potatoes, :as => :potatis
Then in my controller, instead of being able to use the english :potatoe_id, I would have to use the swedish :potatis_id, not good imho.
There’s also a problem with inflection, if the name given to :as is plural (as it probably should be), then the param would have to be pluralized as well (for this to make any kind of sense). Now it gets really icky.