Due to the recent discourse about the proposed rule changes in commander there's a lot of talk about hybrid mana and how it works in the game. I feel many people don't grasp many of the nuances of the situation and use false or misguided arguments.

The Point of Color

Magic the Gathering is a game where you build a deck made of lands and spells. In a game where you choose which game pieces you use, something appears called the Queen problem. The Queen problem boils down to "why wouldn't you run all queens in chess if you could choose the pieces?" and the easiest way to solve it is to put restrictions on which game pieces you can run.

High mana value cards are one way to limit this options by pushing them to later in the game, since you need to build up resources to cast them. Another limiting factor could be making something a Sorcery instead of an Instant, or making a creature unable to block.

Color is another way to limit powerful options on the deckbuilding step. When building your deck, you'll have to balance how many spells against how many lands you are running, and it's in your best interest to run as few lands as you can get away with without hindering your game plan. Color variety works in a similar way. The more colors you introduce, the harder it is to have a consistent mana base.

What this means is that color is a deckbuilding limitation. If all colors could do anything, this limitation doesn't serve to do anything, that's why certain effects are present in some colors and not others. This is where the terms primary, secondary and tertiary come from. Trample is primary in green, secondary in red and other colors can get access to it at higher rarities. If you want a creature with Trample in your deck, you may need to add green or red to your mana base, lowering your consistency.

Yes, color has an aesthetic difference, but its origins are in mechanics. That's why color pie breaks (a color getting access to something it shouldn't) are so disliked.

Hybrid Mana

Hybrid mana appeared first in Ravnica, City of Guilds, and served as a way to give draft decks in a two-color set a way to get useful cards outside of their faction without having to add colors to their mana base. Importantly, this didn't only apply to draft, it worked that way in all other official formats.

With time, the design philosophy of hybrid mana has become clear, the card could become just one of the colors and still be inside the color pie. And of course some cards have slipped, like Augury Adept giving blue a way to gain life, but many cards have broken the color pie over the years and in way worse ways, Beast Within or Harmonize for example.

Hybrid cards also solve a design problem. Let's take for example Wrath of God and Damnation. They are the same card except for their colors, one being white and the other black. This gives white decks the option to run Wrath of God and black decks to run Damnation. But it also makes white and black decks able to run both, letting them being able have greater consistency while deckbuilding. Hybrid mana helps solve this problem, by printing a white or black hybrid card called Wrath of Damnation instead, white decks and black decks still have access to its effect, but decks with both only get to run one playset of the effect.

Commander

Commander is a format with one basic premise, to build around a Commander. This comes from a context of wanting casual games where self expression is the main selling point, with rules like 40 starting life and 100 cards singleton decks being tools to help that play style. This format also limits what colors you have access to by checking the colors in your Commander. This comes from a flavour standpoint, but it also serves to restrict mechanical effects to what the colors you run have access to.

Up until now, hybrid mana has been considered to be both of its colors when applying color identity rules in commander. This means hybrid cards have been locked out of many decks that would have been able to run that same effect if it had been printed as a mono-colored card. In fact, if the approach taken was similar to the case of Wrath of God and Damnation, we would have blue and green decks able to run two copies of Cold-Eyed Selkie, each with a different name, making it twice as easy to find.

The New Ruling

I believe the new ruling on hybrid cards are the best way to move forward. R&D would be free to design cards that could fit either one of two colors without giving decks with both the option to double down on that effect. It would also make commander more intuitive, since most people coming from other formats already expect hybrid mana to be playable in a deck with just one of its colors. I don't think completely new players should start with commander, but even if they do, I don't believe explaining hybrid mana is harder than just saying "if you can pay all of a card's mana costs by paying mana from your commander's colors, you can run it in your deck."

Other counterpoints I've seen online are about how WotC will use this rule to print pushed hybrid cards, but if they wanted to print a card with mass appeal they can already use colorless artifacts, like the One Ring. Many people also fear that cards like Treasure Cruise will be printed on blue or red hybrid, but that would be a color break, which could happen with or without hybrid costs. They can print a red Treasure Cruise whenever they want, knowing that it is a color break. Things like Phyrexian mana or flip cards are also a point of discussion, since they are adyacent to hybrid mana, but Phyrexian mana is more similar to an alternative cost which WotC recognizes as an overstep (and with the explanation from earlier it would still be excluded), and most flip cards I've seen either don't break the color pie of their original color or require paying mana of the new color to be able to transform, making them still not be a color break. Archangel Avacyn does break the color pie with her transformed enters the battlefield ability since you don't need any red mana to transform her, but still, color breaks happen sometimes and if they are truly a problem, they can be banned. There's nothing wrong to dislike color breaks, but we are talking about an casual eternal format with very few bans, color breaks are rampant. And the rules change coming right before Lorwyn is not some conspiracy, same way they would revisit a problematic rule when printing a set that uses it, they want to do the same with the hybrid mana ruling.

Some ending notes: White has been cited by MaRo to have access to additional combats, so Waves of Aggresion is not a color break, please stop using it as an example.