well, you can't really say they NEVER will... i mean look what they did with Wind Waker. the demo at E3 2000 showed realistic graphics, and then Nintendo surprised us all with Cartoon graphics... an M-rated game could happen. i get that the Zelda series is meant fro all ages, but honestly, the Zelda games are never the case. Zelda games aren't pick-up-and-play games like New Super Mario Bros. or Wii Party, you'd have to understand Zelda in order to pay the game and to actually get somewhere. an M-rated Zelda would really bring to the table new ideas that couldn't be implemented anywhere else...
but, it's still Zelda, so of course i'd but it... ^^