I posted this in ATC Humour about 10 years ago, but I think it shows the danger inherent in VMC climb/descent .....
Many, many years ago, an Aer Lingus inbound Manch told Preston Airways he had the BEA aircraft opposite direction in sight and asked for descent maintaining VMC which was given. The BEA skipper said "Aer Lingus , how do you know the BEA aircraft you can see is me?". Quick as a flash, the Aer Lingus skipper said " BEA, how do you know the Aer Lingus that just descended through your level was me ?"