We like it but we go for more days and we love to revisit favorites or go over to the other park if one feels too crowded. It's also nice that the two parks are just walking distance from each other.
We never get the hoppers at WDW though. It's too much of a hassle to go to different parks there IMO. It's a matter of personal preference.
It's not really necessary but it depends on the person. Some like it...some don't worry about it. If you purchase the two day base ticket and you find you want more time, you can upgrade at Guest Services to the park to park and pay the difference.