There's something I don't get...youtube originally started as a means to upload and share video's. Later, as the site became more popular than some television stations, adds ensured that the ones who were most popular got a bit of money from it. And even lots of money if you were popular. The problem with it that was originally small, though, also got bigger: that the used content to a large degree isn't of the creator of the video. It's not that hard to just upload a part or even a complete movie, serie or music track to youtube and watch the money roll in. Except that doesn't happen, as the owners rightfully wave copyright claims at me.
Why would video games be different? Let's plays are fundamentally different than reviews, critiques or other kinds of youtube-vids where only fractions of the game are shown, but get somehow thrown on the same pile in this discussion. And I really don't agree to that.
-the "free advertisement" shit is doubtful at best. Reviews and critiques may help a good game sell better, but let's plays? I've seen let's plays consisting of multiple hours (seen as in 'seen the description'. Not watched them entirely). Are we really to believe that people watching all that, see all the spoilers and gameplay, are going to go out and buy that game afterwards? I don't think so. It's the same case as with movies: show too much of it and people won't buy it anymore.
-this is a relatively new phenomenon. I kind of wonder how cocksure everyone is in their conviction that this is a failed marketing move by nintendo. I'm open to suggestions as to why, but thus far, nobody seems to be able to tell why that would be (in fact, nobody even seem to mention that at least they're not hindering anyone from uploading video's anymore).
-I don't watch let's plays, but it wasn't hard to see that most of them revolve around AAA-titles. In other words: the games that would've sold good anyway. Even more: due to the lack of decent third party titles, pretty much all that's left to do let's plays on are about titles of already very known and established franchises. And it's not like the sales of those titles will go up if there are many let's plays floating around.
-finally: do people follow let's players or rather the actual game being let's played? I would think the latter. So let's say there are 10 let's players in total. 9 of them are disgusted by ninty's policy and do stuff on other franchises. The tenth does a let's play and gets half the money. But because not that many people make these let's plays, he'll end up with more hits (and thus more money). So in the end, it scares away those who only do let's plays for money. And it may be black-and-white'ish, but...how exactly is that a bad move?
Reviews and critiques (and satire/parody*1 and eduction) would be exemptions from copyright though so that does change things a bit. From what I have seen most people seem to be using some kind of nebulous "but they have to do work on top of it" kind of argument, one that has precisely zero basis in most IP law. Copyright would seem to be the main thing companies are using, though some have dabbled with some trademark stuff for some aspects of it (I reckon Microsoft's recent policy on the matter has a lot of trademark stuff underpinning parts of it -- http://www.xbox.com/en-us/developers/rules if you had not seen it). Let's plays would not seem to be part of that, at least not by default. "fractional amount" means surprisingly little in IP law, though I would agree that if you potentially have to justify each use then it would be easier to justify if you did not have the lot there -- there is nothing to say a critique has to be shorter than the work in question or can not cover the entire work, the key word for most of that side of law would be transformative, something I agree most let's plays are not.
In the original Nintendo vs let's play threads I made comments to the effect of "end of the gravy train" or "the wild west has been tamed" and I stand by them here. That said I would have been content to have it remain as something of a legal grey area.
*1 was not in some (many?) EU states until later last year, had been in the US for a while.
Let's plays as advertisement is probably a whole discussion unto itself. Alas the only real marketing/sales data I have seen shared for games was the demos make sales go down idea ( http://www.pcgamesn.com/jesse-schell-releasing-demo-harms-your-game-sales ). However similar things are studied in films, TV and books, possibly with some analysis for piracy ( https://torrentfreak.com/director-won’t-think-less-of-you-for-downloading-on-bittorrent-080116/ is an example and torrentfreak has endless examples of similar things from various directors/producers/people responsible for such works).
The main thing seems to be it very much varies with your projected audience and the relative budget/reach of your work. Various high end things being troubled and a lot of low-mid range stuff often being boosted. Now I am hesitant to link things too heavily if games are supposedly still going to be getting the vast majority of their profits within the first few weeks of release (even if I hold that to be an accounting scheme a la Hollywood accounting most of the time, especially in this time when everything is available as a downloadable game rather than just a select few staying on shelves, or returning to shelves as a greatest hits).
With games you say about spoilers and that does seem to change the attitude devs take on a per game basis -- seeing someone play minecraft for four hours is probably not going to spoil it for me, seeing someone play through some story based game without choices may do this, or at least sate me until I see it in a bargain bin three years from then/it has a nice GOTY edition the next year.
To this end AAA, even more than my usual issues with the term, may not be that useful a description here, not to mention I am not sure how many of them revolve around this categorisation.
There have been various games that rose to prominence after let's plays were done, because a handful of the people doing them were popular*, whether that is because of that, because they formed part of a grass roots effort or utterly in spite of that remains to be debated.
On the flip side I have seen let's players where the players were exhibiting what I can only describe as infomertial levels of stupidity (on top of being painfully unfunny) and trademark law does include such lines as "general public likely to be confused". If certain types are likely to see your work cast in a negative light then it might be worth sacrificing this for the greater good.
*I do not know how much you know of slightly older UK radio but there was a DJ called John Peel, being played on his show meant a lot more than most other things you could do. There are similar examples of such people in all sorts of other areas, even today with culture/entertainment being as unfocused/diverse as it is compared to previous decades. The term behind it being "tastemakers", however I am not sure it is the most fitting here.
"do people follow let's players or rather the actual game being let's played?"
I covered part of that in the previous paragraphs but I think this might be an example of the internet loving its "fairness", all while completely misunderstanding the concept.