I've just read your post over in the "Ideas Section", and I must point out that you have made some points and queries that require some clarification.
I will try to answer your questions in the order they were asked.
You first asked, "So what's the point of the included SD slot and USB port?", Well, the Now TV black box and the Roku 3, which the black box is cloned from, are made in the same factory and it would not be economically viable to remove those ports.
You also mentioned, "...it seems bizarre to sell a product with ports that can't be used - especially as some adverts for the device specifically mention these ports." Yes, it does seem bizarre but you have to remember the black box is sold at a cut down price and is primarily a vehicle to deliver Now TV content and nothing else. As for the "adverts" they are mistaken because the writer did not check the specs just copied the Roku 3 specs, which has fully functioning ports.
Finally, you suggested, "So I vote for a Plex or similar media player to be added ASAP." This is never going to happen, i.e. such a facility would introduce more"user choice" to the Now TV black box and, as such, take custom away from their offerings.
I think that they're deliberately leaving this feature off their boxes as it would reduce the income they make from their Sky passes and monthly subscriptions. If there were a media player it would mean that many people would simply download movies and store them on a memory stick and keep it on their NowTV Stick. I belive this is why in their latest releases of the NowTV Stick they have removed the developer settings pannel.
You know, I had to re-read my post from January 2018 to reacquaint myself with this discussion.
First off, I started out with the Now TV white box and graduated to the 2015 black one, so I have been aware of these devices limitations. However, when Now TV suddenly, and without warning, banned the developer's option, and gave a highly bogus reason for that ban, I bought a Roku 3, which I still use today.
From a personal perspective, I believe there is no point arguing with, or against, some of Now TV's unpopular policies simply because they are not going to change them. The best thing any of us can do, rather than getting bent out of shape, is to seek alternative and less restrictive methods to consume Now TV's programming, such as the Roku range of streaming devices.
I see Roku and Hisense have teamed up and are bringing their OS to the UK at last
It's good to know that Roku will be licensing it's OS out to TV manufacturers, especially as my TV is coming to the end of it's shelf life. However, I do prefer to buy Samsung TVs rather than anything else, so I'll soon be on the look out for a Samsung running Roku's OS.
PS. As you can imagine, my fall back position will be a non-smart Samsung with a Roku streaming device.
That TV may not have been gracing @ukbobboy ‘s no doubt equally elegant home....😛