Commissioning of ILEM candidates

Tonight, at St. Rose Parish in Freehold, we had the commissioning of the 6 recent graduates of our ILEM program.  ILEM stands for Institute for Lay Ecclesial Ministry.  The ceremony was conducted in the context of an Evening Prayer Service....the Vespers of the Liturgy of the Hours.  The musical leadership, choir, cantor and instrumentalists were outstanding.  The whole service reminded me of the celebration of Sunday Vespers with the choirs out at Notre Dame U. 
I did, however, note some "flubs" in the service.  I was not pleased that the priests of the diocese were invited to wear cassock and surplice to "join in the procession of clergy" for the Evening Prayer.  Theologically speaking, this was to be a celebration of lay ministry.  And so I ask a simple question:  "Why set the clerics apart from the lay faithful at a celebration in which all have legitimate parts?"    They are not concelebrating, so I ask the question: Why do the priests have to vest in cassock and surplice to attend Evening Prayer?

Looking for answers!