Unfortunately
none of the perks you mentioned (other than it being free to sign up) apply to the MGM Player's card

I get some discount offers through email, but that's because I'm on the hotel's mailing list, not because of my card. You get no free play, no discounts on dining, and money that you spend on food, drinks, and rooms are in no way tied to the card. I don't have a Wynn card. If they still offer a free buffet I'd look into it! As far as "every casino you enter", all of the MGM hotels (Bellagio, Aria, Mirage, Luxor, MGM, NYNY, etc.) use the same MGM players card, you wouldn't need one for each casino.