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The physical-special split is the most overrated feature Gamefreak has implemented. On paper, it sounds great but in actual gameplay it has little impact as for every Pokemon that greatly benefited from it, there was also a Pokemon that was significantly hurt by it, pic related. It was also the beginning of types losing their identities:
Rock moves are inaccurate but have a chance of a good effect.
Fighting moves are strong but have some form of a drawback.
Dark moves are weak but disrupt the opponents attacks.
In addition Gen 4 was when Pokemon started receiving way to many coverage moves, therefore subverting the game's philosophy of needing to have a diverse team of Pokemon to cover up each ones weaknesses.