You are able to record full sessions since you joined the world/server, and it ends once you disconnect from the world/server, so there is no need to have a feature to have it record ahead of time, and you can crop your replays afterwards anyways. And the option to disregard or save a replay could be a nice feature, however it isn't anything that should be highly focused on, since you can just as easily delete the replay footage when you disconnect from the server/world, and then go from there.