I’m sure this has been asked (and answered) before, but my Google-fu is failing me.

I’m sure this has been asked (and answered) before, but my Google-fu is failing me.

I’m sure this has been asked (and answered) before, but my Google-fu is failing me.  What is the reasoning behind the “only one per class in a party” rule?  My friends and I are looking at DW and there may be two players that want to play the same class.  What’s the harm?