I’m going to move this topic to the Contribute > Feature category. I think it should be possible for users who do not have permission to send PMs to other users to still be able to message a group.
My reasoning for this is that the easiest way to setup a support system on Discourse is to create a custom group to answer support requests, then allow users to send PMs to the group. Currently this functionality depends on having the enable personal messages setting enabled. Even with that setting enabled, only users who meet the min trust to send messages setting requirement will be able to send messages on-site to the group. I don’t think that setting up this type of support system should be dependant on allowing users to send PMs to other users.
One possible way of dealing with this would be to add a min trust to message group site setting. That way regular PMs could be disabled for regular users by setting the min trust to send messages setting to Staff (or 4:leader), but all users on the site could be allowed to message the support system by setting the min trust to message group setting to 0: new user.