At this time I am playing solo, but I frequent open as well. Sometimes I need space from the ats who only target players. And I understand the arguments for playing in open, such as, other players being a greater challenge than npcs. However, that is also a disingenuous argument.
Frequently I've seen in playing open, and in forum posts, is players boasting about and only going after low end new players, or easy targets. That's not a challenge. In CZs I've seen player pythons, fdl's, and condas go after singular eagles and Sidewinders only bc they were cmdrs, when not far away there are competing npc condas and pythons with far higher credit and rank scores.
An interesting experiment, and I'm not recommending the game change to this, would be to see what would happen if you had no way of knowing if that target is a cmdr or not, and how likely griefing would occur.
The only real argument I have seen by those that believe it should only be open is for the opportunity to blow up other players for no other reason to grief them. Also to meet them....some are friendly. Not everyone is an at.