Proposed benefits:
/ * more verbose, but you can see more clearly all the "stuff" your method is using
/ * easier to re-use partials/helpers, since they're not coupled to instance var names
/ * easy to tell what data you need loaded to be able to use a helper/partial


/ top_file.html.haml
= render :partial => 'user'

/ _user.html.haml
= user_full_name_markup
= render :partial => 'user_friend'

/ _user_friend.html.haml
= @friend.name

def user_full_name_markup
  <em>@user.last_name, @user.first_name</em>
end

vs

/ top_file.html.haml
render :partial => 'user', :locals => { :user => @user, :friend => @friend }

/ _user.html.haml
= user_full_name_markup(user)
= render :partial => 'user_friend', :locals => { :friend => friend }

/ _user_friend.html.haml
= friend.name

def user_full_name_markup(user)
  <em>user.last_name, user.first_name</em>
end