This might be one of the most important things I've written.
For the purpose of this post, we need a definition of "diverse" as it pertains to Yugioh. So here's one that I think is fitting:
A diverse format is one in which there are not only one or two decks decks that are overwhelmingly more powerful or successful than most of the others.
With the banlist now only four days away, we're starting to see the
time old debate of diversity popping up more and more. Nearly every
player agrees that it's about time for the Dragon Ruler deck to go the
way of the dinosaurs, but few completely agree on the impact it's had on
the game so far. Everywhere I go, I constantly see people spout their
ignorance about which type of format is better - diverse or
I think that this is an argument with almost endless outcomes based on the way you look at it. So, I want to be clear with you guys by telling you how exactly I'm going to come at this question. When I personally think of what's "better" for Yugioh, I think of what makes it healthier. There are positive aspects to both types of format, but what I think is most critical to this argument is the simple fact that non-diverse formats tend to favor skilled players. The lack of playable decks in a non-diverse format will emphasize a player's control over the mirror match, which is where skill and experience plays a more direct part in how often a player will win.
Also, and perhaps even more importantly, facing the same deck over and over erases the possibility of losing simply because an opponent's strategy is favorable against yours. When a deck simply has a good match-up, it becomes possible for opponents with comparatively less skill to defeat better players. A person who just started playing two months ago that picks up and learns an Evilswarm deck has a decent chance of defeating a well-known player like Billy Brake when he's playing Dragon Rulers, simply because the match-up is just that heavily weighted in the favor of Evilswarm at times.
I'm not saying that every person that ever wins a match in a non-diverse format is always better than their opponent (or that those formats have no luck or variation whatsoever), or that diverse formats are completely void of any good players topping premier events. It's just that it's more likely for the better player to win when both players are being tested on their knowledge of the same cards - it makes it a lot easier to compare the skill levels of two people. And in most match-ups, this also helps erase the possibility that one player has an advantage simply because of the nature of their decks. Decks like Dark World and Frog Monarchs are obviously exceptions, but Dragon Rulers are most decidedly not.
So once we take this into consideration, does that mean that non-diverse formats are better for the game? Honestly, that completely depends on what you mean by "better." If your idea of a better game of Yugioh is one in which only the best players have a good chance at winning events or being successful, then fine - that's what you believe. (And I think there are good ways to back that belief up, but that they aren't as good as the argument I'm about to show you.)
But personally, I think the opposite is true. While it's certainly frustrating to lose to someone worse than you, and diverse formats boost the chance of that happening, I think that this is better for the health of the game as a whole. But why would I think something like that?
Let's look at two different local card shops for a second:
Card Shop A exists in a time where Yugioh can typically only be won by the most skillful players. But since most people aren't that good at Yugioh and thus can't top reliably at the local tournaments, the usual attendance for a weekly tournament is about 8 people.
Card Shop B exists in a time where Yugioh can be won by nearly anybody as long as they have a decent connection to the deck they're using and don't make any gigantic misplays. Since most people have a chance at topping locals, the usual attendance for a weekly tournament numbers between 35 to 45 people.
If you think that Card Shop A is healthier for this game, then I'm sorry to be the bearer of bad news, but you're wrong.
When I personally think of what's healthy for Yugioh, I think about it in a very literal way - what conditions will make Yugioh a self-sustaining franchise that can survive the test of time? And I think that, more than anything, what dictates this is the size of the playerbase. If nobody plays Yugioh very consistently because there's no point in getting their asses kicked all the time, then what motivation do these people have to buy any cards? And if nobody is buying cards, then how is Konami supposed to justify producing a game that earns them no revenue?
(Also important is the fact that players with less experience tend to spend more money on sealed product, which makes them much more important to Konami's bottom line than your typical, tournament-tested veteran.)
I understand that these are all somewhat extreme examples and situations, but they help to illustrate my point. I'm not trying to say Yugioh should become a big sackfest, or that being a good player isn't worth anything - nothing like that. (In fact, no matter how many caveats I take note of here, I'm sure somebody will still think of something else inaccurate that can be drawn from this assessment.) It's always good to reward the people who have worked hard to get good at something. But generally speaking, Yugioh will survive longer with more people playing the game. And most people aren't motivated to play a game that takes too much work to be successful at, especially while they have to constantly feel the sting of defeat. Humans are naturally lazy.