From a security perspective, allowing all incoming connections by default is unnecessarily exposing devices to a hostile environment. The average Joe isn’t going to understand the risk unless somebody explained it as “it’s like posting your home address on 4chan and hoping nobody manages to pick your front door lock,” and they’re likely never going to take advantage of the benefits that come from having their device be globally reachable.
Another benefit to not having to deal with NAT is that you can actually host services using the same protocol (e.g. HTTP) on multiple machines without having to resort to alternate port numbers or using a proxy with virtual host support.