No they are not necessary.
We had four days during our last trip and didn't hop. I know the parks are very close and hopping is easy but from my son's math, and yes he really did this, he figures we spend about an hour each time we hop from the time we leave until getting on another ride. I find we enjoy the parks so much more if we just stay put. For years we missed out on some great things because we would hop back and forth doing mostly the highlights. When we don't hop, we get immersed in the magic and get to really explore. To each his own and I know I am in the minority. We have APs this year so we'll have the option but I doubt we'll use it.