I felt compelled to respond, even though I've more or less exhausted my arguments already.
So, I really want to take a stab at that "competently built" argument. To summarize what is seems that people have said, the idea of this Pokemon was to see how Sketch works on a competently built Pokemon, and that without Substitute, the Pokemon is not competently built.
Incorrect. I said that Substitute is an integral part of most competently built Pokemon, not that competently built Pokemon had to have Substitute. There is an important difference. Nearly every Pokemon in the game has Substitute, so it seems natural that one of the questions we should be asking ourselves is how, exactly, does Substitute affect a Pokemon that can pick almost any move? For all our theorymon, we do not exactly know the answer to this, hence why I advocate the "let's find out" approach.
I just cannot believe that this is the truth. Wobbuffet doesn't get substitute, or most other moves for that matter, but I doubt people would argue that it is not competently built. In fact, I really don't see how an argument like this is anything but a flavor argument. Most Pokemon get it, so most Pokemon have a bunch of options thanks to it, therefore it is required to be competent? How does that logic even work? Really, there is nothing in that idea that give me even the slightest inclination to think Substitute is necessary.
Wobbuffet is competently built in the same way that a plastic tower held up by a helicopter and an iron cable is competently built. It's not likely to fall down, that's true, but that doesn't mean that it must therefore be an example of excellent design. Wobbuffet itself is the product of a very lacklustre Pokemon given two important tools - Shadow Tag and Encore. As has been demonstrated, Encore broke Wobbuffet's usefulness by a not inconsiderable amount. I do not believe this would have happened to any Pokemon I would describe as "competently built".
You appear to be labouring under the assumption that the entirety of this argument is based around the principle of "everything else gets it, so Necturna should get it too". This is not the case. The argument is that Substitute the move is an integral part of the way that competitive Pokemon is played, and in order to simulate the effects of giving one Sketch move to an "average" Pokemon, it seems unreasonable to discard Substitute because of fears that it may be broken, when we don't know if it will be yet.
However, this is not about whether or not it is necessary, but about whether or not it should be allowed. On the one hand, yes, it will certainly increase the versatility of Necturna, which is one of the interesting parts of this CAP to begin with. However, on the other hand, it has the potential to put Necturna over the top. I won't claim that it would make Necturna broken, because I have no idea if it would. But the potential is certainly there. Additionally, I actually think it distracts from the concept.
I am fairly certain that every single CAP that has ever been made has been called broken at some point. I will not argue the point, because I do not know what the possible consequences of Substitute are either.
I believe back in the concept stage, one of the ideas for a project (reachzero's, if I remember correctly) was all about Substitute. Simply put, it is one of the most powerful moves in the game, and studying it would certainly be interesting. In fact, I believe we may still be able to learn a lot about this move and its influence on the game with this CAP, even without it as a central concept. However, by giving Substitute to Necturna, we are almost guaranteeing that it will be on every offensive set, and also we would be further increasing the offensive bias of Necturna. I believe we would actually learn a lot more about both Sketch and Substitute by not giving it to Necturna. As I said, Substitute is one of the most powerful moves in the game, and I can't think of any better way to study the effects of it and of Sketch then by disallowing it. If it truly is the ultimate move, we would learn about that by having people sketch it.
I don't believe that anybody has claimed that Substitute is the ultimate move, and to be quite frank it is irrelevant for our purposes. Nobody is going to Sketch Substitute if it is not allowed, unless of course it's for a Leech Seed set, in which case you will be forced to forgo Sacred Fire.
I am not entirely certain how you are arguing that we will learn more about Substitute by ignoring it completely. Whether or not Substitute is there, we can make fairly accurate predictions about what will come to pass - we will still have a wide variety of offensive sets, and support sets, but some of them will have Substitute. If we have Substitute, then yes, we run the risk of making our sets offensively biased. If we do not have it, then sets will still be offensively biased, but with three attacking moves and one less turn to set up in.
The distraction point is certainly an interesting one, and I'll admit I find it rather amusing that the same reasoning I have used before, I am now arguing against. Well, I suppose the reason it has not occurred to me that Substitute is distracting is because it is, in my opinion, a move that is represents a universal strategy - that of mindgames. Substitute simply works in tandem with other moves to produce new strategies, and it is a technique that nearly any Pokemon can employ. I am beginning to repeat myself now, so I shall stop.
I will say here, very briefly, that I think we ought to be careful about how far we go in terms of "manufacturing" Necturna. It is already doubtless that we have tried to shape Necturna to our vision of how it should turn out, but remember that we are supposed to be learning something here, which will not happen if we take out everything that has the slightest chance to derail the CAP from our immediate expectations.
The point here is that we are learning something regardless of whether Substitute is there or not. The difference is that what we are learning essentially refers to two different Pokemon. I am of the opinion that the way that the move Substitute interacts with the rest of the Pokemon's movepool is a part of how Pokemon the game works, and that to not include Substitute is to remove an element of doubt that would be a factor for almost any other Pokemon. I think I am right in saying that you believe that this does not matter, and that only what we have specifically created should be judged based on the Sketch move, such that the traditional Pokemon build is irrelevant. This is a perfectly fine stance to take; however, there is no direct way to compare the two views objectively.
Sorry for the incoherent response - it's late here. I await the storm of flaming and walls of text in the morning with bated breath.