You can't make an "Only In Progress Feature", because if every tank used only that feature no party could ever form to begin with. So you're never going to see that.

SE intended Join In Progress as a means of helping out parties that had lost a member, so that they could progress without needing to start over, not as a shortcut. To a certain extent they'll accept this happening, but there's always a breaking point. If people keep dropping and queuing all over again, SE probably will take measures against it.