Most languages which allow for function head clauses also have case or switch statements. So you can use both. I like the pattern when there is a corner case that needs quite a bit of logic but it's not very important for how the code usually works, and you won't want an indent level on it. For example:

    function(SpecialCase ...) ->
       ...
       handle_odd_special_case;

    function(DefaultWhatever ...) ->
       ...
       main_business_logic.

This is more cleaner _sometimes_ than say

    function(...) ->
       case Arg of
           SpecialCase ->
               ...
               handle_odd_special_case;
           Default ->
                ...
                main_business_logic
    end

Now imagine another level of case statements and all of the sudden the main path of the algorithm is squashed to the right with only a 40 columns left.