Yea you are right about the betas as being only a portion of the game. But the betas are too far apart to justifying getting ps plus. For the real reason to get ps plus is to get the free games or the discounted games that you would normally wont get anywhere else. This is why i said i rather get the full game.
Beta's are the least picked reason to get Plus. I actuallupy think its one of the last things listed as the perks of having a subscription
Beta's are simply used as a marketing tool to garner hype for a game, as well as test the stability aspects of the game.
Not every game gets a community based beta either.