Consider you have two of these things close together:

  [10, 11, 12].map{|x| x.to_s(16)}
  [:baz, :bam, :bar].map{|x| x.to_s(16)}

At least for me, I tie the 'x's together in my head; so I have to concentrate to realise that there are two different 'x's.

I've not done any formal research into this, it would be interesting to see whether there's actually a perceivable difference between the two styles.