I can see from your description where claiming per topic may be desirable in some circumstances and/or for some forums.
On the other hand, we run into the situation where we get surprised by a flag being assigned to someone who claimed a flag days or even weeks earlier. That moderator may not be active when the new assignment is made (eg. vacation, etc.).
Perhaps the best solution would be to have an Admin Setting that let’s the admins choose whether claiming is done on a per topic or per post basis.
We don’t want to turn claiming off entirely because we find it very valuable in preventing duplication of effort in researching flags and queued topics before acting, and preventing one mod from acting on a flag while another mod is considering it.