OP, if your question is honest and you are not just some brand new poster trying to stir up an onilne argument, then I apologize. But really, what kind of response do you expect posting a question like that on a internet posting board full of WDW fanatics. I mean sure, you'll find your share of folks here who favor the Magic Kingdom the least out of the 4 Disney parks, but I don't think you'd be able to find a single person on this board that would tell you that going to the Magic Kingdom was a waste of your time.
Are the lines long? They can be if you don't plan ahead and don't use fast-passes. I always say with complete honesty that I don't wait in Disney lines longer than 20 minutes, ever.
Do you sometimes spend a whole day there and not get to see everything? Again, this is a good possibility, especially in high seasons when the parks are full. However, have you EVER gone to a theme park for one day and seen everything there? I haven't. I mean, have you ever been to Cedar Point? I once spent 4+ hours in line there for ONE RIDE! And it was terrible. Anyone who goes to a park thinking they'll get everything done in one day is fooling themselves. I usually schedule at least 2 days in the Magic Kingdom during a WDW trip, and I am not someone who cares if I miss a ride or two.
Do you need to experience it? As what, a part of a Disney vacation? Yes, I would say a trip to the Magic Kingdom is an integral part of any WDW vacation. As part of a trip to Florida in general, but not specifically WDW? Maybe, that depends on your family's preferences. As part of living life and experiencing fun things in general? I'd say yes, a lot of others would say no.
Basically, OP, if you're seriously planning a trip to WDW and you're asking yourself whether you want to visit the Magic Kingdom, I'd have to ask you, "Why visit WDW at all?"