I think there are positive benefits associated having new members wearing the letters-- it promotes your group to the outside world and gives them a sense that they belong-- which translates to personal pride and investment in the group, wanting to be involved and learn the ways of the group.
Members are educated on the importance of respect for the letters and public perception of those who wear the letters.
I'm not jumping on any org or person who thinks differently-- that's their belief. If a group is going against what their HQ has decided, I agree-- they should follow the inter/national protocol.
But from a PR and acclimation perspective it makes sense. And new members can be given an "open meaning" for the letters-- the same as you give them an open motto.
ADPi's open motto "We Live For Each Other"-- great promotion of our sisterhood!

My chapter tells non members and new members that the letters stand thus:
Alpha: the first letter of the greek alphabet. We were the first secret society for collegiate women
Delta: Like a river delta, we are ever flowing, growing and changing.
Pi: This is also a mathematical symbol for an infinite number. ADPi is eternal.
Based on the line of reasoning that a new member shouldn't wear letters until they know what they mean, does this mean they should be able to call themselves a new member at all or participate in any activities with the sorority?
ETA: I had a long-winded example in another context, and then realized "Who cares?"