"Yes, and..." improv is a fun and powerful way of crafting stories. However, it is defective when applied to an RPG focused on exploration and dealing with external challenges. Not moving board pieces around and being a passive observer, but making difficult, meaningful choices.
Also, you can't write Shakespeare using improv. The product of improv is its own thing. Really, part of the reason it works is because of the spontaneity. Watching and rewatching improv on video does not have the same impact. Knowing that nothing has been decided in advance is what makes it work. Movies written with improvised dialog feel rather different than traditional poetics.
Same thing for an RPG; the undecidedness defines the genre. However, the method of making events significant in the moment is different. "Yes and..." is one possible technique, but "There is a gaping 10' pit in front of you, deal with it," is another. I can appreciate games where the adjudication and storytelling are widely distributed, but I do not prefer it. Whether as GM or as a player, I prefer an imaginary world that is stubborn, not pliable to a player's wishes.
I will start by saying that I humbly disagree with some of your points regarding improvisation techniques. You say you can't write "Shakespeare" using Impro? Well, I can't speak for how Shakespeare did it, as I am not him, and I can't write Shakespeare for the same reason (no one can)
*. However,
I have written many plays, some of them based in part on improvised work with other actors.
All of them my writing incorporated many of the impro techniques I had learnt on some level. Just because I am not getting up on a stage and acting it out doesn't mean I am not improvising. I am just improvising with a pen and paper. That is not to say I don't edit and refine later on, when it becomes the script, but the core of the piece is there.
The same goes for Role Playing Games. I am not saying you should put away your dice and maps and just improvise everything. We are not playing "scene from nothing!" I am saying that some of the techniques might well work for your players and the style of game you are trying to achieve. Naturally not all the tools will be appropriate for every situation (or every group) and you don't have to improvise everything.
DEFCON1 was just offering one tool from the improvisation toolkit that can be used by GM's and demonstrating one way of implementing it. It's easy to show how that implement as described does not apply to all situations and then dismiss it as somehow defective. But I think it is more robust a tool than that. I would disagree that it doesn't have a place in games based on 'external challenges and exploration" unless you are suggesting that the GM uses the "Yes, and" tool to the exclusion of other approaches and without exercising his better judgement. Which would indeed be silly. After all, "Yes and..." is just a training exercise to get improvisers to understand that you need to do more than just accept an offer. You need to build on it, too.
I am not sure I get your point about shuffling pieces etc - I think you are referring to chessboard theory and commenting on the metagame level at which DEFCON1's example is pitched(?) All I can say in response is that I think that "Yes, and..." as a tool can be applied with various degrees of subtlety to different gaming styles. (Also, frankly, I have seen many meaningful and difficult choices played out in games that were strongly impro based)
Obviously, a single rote response can't apply to all situations, but IMHO "Yes, and" is a lot better as a default position to have than "No", until the point where you meet an obstacle. Despite what DEFCON 1 said about most GM's being comfortable saying yes, all too many GM's have "No" as their fallback position. And it is often not because they have determined the facts earlier to be otherwise, but because they are afraid that saying yes will take them into territory that they have not planned for.
I do agree that it is important for there to be at least the illusion that the world is not just a solipsistic reflection of the player characters, and that it is good when that there is more going on in the world than what the players are going through. I also acknowledge that gaming is a different art form to pure impro, and that the synergies can only take you so far. However, there is also a lot that can be drawn from
good impro training
** that can significantly improve one's game, even if it isn't entirely improvised. There is no reason why you can't prepare your game, establish immutable facts about your game world and, yes, even say "no!" when you need to (constructive blocking) while managing to incorporate solid impro techniques.
Basic impro tools can give you have a better understanding of spontaneity, narrative etc, and when you have a command of the basics you are are often more able to create details on the spot, dramatically improving your capacity to GM 'on the fly' (pun unintended). The extent to which you allow the game to be a free flowing collaboration of ideas between you and your players vs a more traditional gaming experience where the GM decides most things, mostly in advance and is the sole arbiter of everything is up to your gaming group. There is obviously a wide ranges of gaming styles in between, and naturally YMMV, but that goes without saying, n'est-ce pas?
*btw, it's actually not that hard to rhyme in iambic pentameter after you have been practicing for a while. People in movies don't because they think it is more 'realistic'. Whether or not they achieve that goal is another discussion
** I am thinking more Keith Johnstone than Viola Spolin here.