The problem I have is not that I don't think training is essential to safe firearms ownership, it's that firearms ownership is tied (this is my opinion) to one's right to their own life. I believe that you completely own yourself, free and clear. You are also completely responsible for your own welfare. As a result it is your right to defend your life. Firearms are bar none one of the most effective tools for self defense when faced with a direct physical threat. Therefore I support the right of all people to utilize that tool in their own defense. So while I believe VERY strongly in the need to train with firearms and gain proficiency with them if you choose to own them I have a very hard time making that a requirement.
A second, and significantly less concerning issue for me is that training, licencing, certification requirements could be used to deny "undesirables" a right to firearms. Much like a poll tax or voter competency requirements limit a person's ability to participate in their own government, firearms licencing and certification limits and individuals ability to defend their own life and potentially limits who can defend themselves against a tyrannical government, which is the basis of the second amendment.
If there were basic firearms safety courses required in public schools I feel we could accomplish everything that licencing sets out to accomplish while not providing the possibility that certification and licencing could create limitations on who is allowed to posses a firearm.