Also, only Drizzle is over centralizing. Sunny Day sucks, the only Pokemon with abilities that enhance it are grass, who hate fire attacks. Sunny Day also lacks any sweeper comparable to the majority of the Swift Swimmers. No Sunny Day booster pokemon is anywhere near as powerful as Kingdra. An instant Sunny Day is no real problem.
As has been said repeatedly in this thread, Sunny Day was tested along with Rain Dance. Both proved overpowering to the DPP metagame.
This was the test, I believe, in the event you're interested in reading up.
Grass's weakness to Fire doesn't matter much when they can overspeed and hit it with various super effective attacks (Ancient Power/Earthquake/Hidden Power), plus the always viable Sleep Powder for anything too troublesome. Even the slowest viable Sunny Day sweeper (Tangrowth) has 100 base speed in the sun. Additionally, any fire attacks thrown at Suny Day Grass sweepers can be eaten up by Flash Fire Pokemon, who are already boosted by the sun.
It's not nearly as effective a strategy as Rain Dance, but it has it's uses. It's mostly limited by the fact that Water is one of the best types offensively and defensively in the game, and Rain gives STAB+WEATHER+DOUBLE SPEED whereas Sun has to deal with two lesser types and only getting two out of those three.
And what has to be understood is that the weather itself isn't broken (as it stands now), but merely the ability to cause the weather instantly upon switch in, especially since the abilities have no time restriction other than actively changing the weather. Which is why the moves themselves, Rain Dance and Sunny Day, are still fine.