There are two main problems with your idea.
First of all, you can't benchmark a game without actually having a playable section of the game, which renders the whole notion useless, because the developer might as well release a demo instead.
Second of all, if the developer is stubborn enough to release a benchmark anyway, then you have to face the fact that a benchmark is not capable of engaging the player. Assuming that the developer provides an objective and accurate benchmark for their game (which is assuming a lot), the only thing that shows is how well the game will perform on your machine, and it offers no indication of the game's story, how fun or scary the game is, how well the mechanics are designed, and countless other factors that would otherwise help people determine whether they'll purchase a game or not.