Agreed. Aside from that, premium membership is about 70 USD a year. Out of that, the premium member gets back all but about 5 USD in perks (512 m parcel, 1000 L bonus, 300 L per week). What should really be expected in return for 5 USD? For FAR less complicated software than SL it is common to pay in excess of 100 USD for 'premium' type subscriptions.
As long as products always include the user name, as long as payments can always be tracked back to the user name, and as long as ARs will NOT result in banning until evidence tied to the user name is investigated, then the security issue is fully addressed, even for merchants. I am concerned about that last one and whether LL has any intention of being that thorough.
I do agree with others that display names that are CHANGEABLE will increase the level of confusion, especially for new residents but for me too. I'm too old to remember that Harry was George was Tom was joey312334. With the introduction of a separate changeable display name, I'm going to have to remember both a user name and a string of everchanging display names.
I would much prefer the display name for a given avatar not be changeable. I would be less concerned about whether they have the same name as me, if the first display name they choose is the only one I have to remember over the long-term. As others have suggested, a titler attachment seems more appropriate for those that want to change names frequently ... or an alt for that matter.