The thing is half the time they watch it because EVERYONE tells them its better then Naruto or Bleach or whatever anime is their favorite. Then they go watch it because they want to say they have watched it and prove that it isn't better. They keep that bias in the back of their mind the entire time and will never try to accept that it is indeed better. Oda doesn't need fans like that. Now, if they become curious on their own accord and watch it. They will think it is amazing and not have that bias. Those are the people that should watch One Piece. People that bash it will never try to watch One Piece without that bias. That is why I say they shouldn't (and a few other reasons I don't want to discuss here).
Look above. People should watch One Piece when they feel like it. Forcing someone to like something they don't want to will just make them resent it more.