Why is a default value needed?

[cserb]

I don't really understand the necessity. Even more it seems to be a bad place to define default values (unless required by design)
Also I want to do something like:

if  !options.string["foo"].nil? && options.string["bar"].nil?
  puts "If you pass 'foo' you also have to pass 'bar'"
end

or just simply checking if a param was passed or not