Well they do something different. It's hard for units to get through to the front of the battle, so less units of you are attacking, if the opponent has more units attacking he will kill more units for less of his.
The difference however is marginal, maybe 5% of all your kills take advantage of being lined up better then your opponents. So it doesn't really matter, but could be important at some moments.
Next to that, if you line up all your units wide like this:
xxxxx-xxxxx-xxxxx-xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
And I line up deep like this:
xxx-xxx-xxx-xxx-xxx-xxx
xxx-xxx-xxx-xxx-xxx-xxx
You have a great change of encircling me, and thus slaughtering me