I made a simple plugin to explain a concept of render-filters.

merb_render_filter

In your controller,

   1  class Posts < Application
   2    before_render :set_title1, :only => :show
   3    before :set_title2, :only => :show
   4  
   5    def show(id)
   6      @post = Post.get(id)
   7      display @post
   8    end
   9  
  10  private
  11    def set_title1
  12      @title = @post.title # <= you can access to @post here
  13    end
  14  
  15    def set_title2
  16      @title = @post.title #=> @post is nil!
  17    end
  18  end

Without this plugin, you couldn't access to instance variables from before-filters. Of course you can prepare @post in the first before-filter so that you can access from other before-filters. But why Merb has the action-args? They are ignored in such case. This was the problem I wanted to solve by the plugin.