I'm not sure why anyone wants this in the first place, but titles or subjects in replies just isn't a desirable thing, in my opinion. How do I get rid of them?
Someone else mentioned that changing to threaded topics did that, but it didn't for me.
You could customize the layout of your posts. If you're familiar with coding (php/HTML) it's a relatively easy thing to do.
The reply title field is designed to be shown on lists of posts like Recent Posts Widget, Recent Posts Page, Profile Subscription, Profile Activity, Recent Posts under the topic in Extended Layout and so on. So, there is not any inconsistent. The post title field is used in lots of places, but in the topic. There is no mean to show the same title dozens of times in the topic. In 99% cases, people don't change post title, and it's the same. That's why we don't show it in the topic.
But there are many good reasons why we show it in the above-mentioned places. The first reason to have a link titles to that post and the second to have a bold/normal title to show unread/read status of that post. All these reasons are meaningless in the topic, that's why they are not shown there.
Thanks for the replies!
Being relatively new to WP, and WPforo, I didn't realize that different forums might have different layouts. After creating some more, they were showing the threaded display, with no "title" for replies. The first one I had created was inheriting the extended layout from the container, which is why it wasn't changing for me.
So, all good. Though ideally I might like to use extended without reply titles. I dislike changing code or CSS, because it makes updates a pain. So perhaps an option for a future version?