I guess it comes down to this:
Are you willing to pay $20 to be able to ride at least 25% more rides than you would without MP?
Since you can pull every 90 minutes as opposed to every 120 minutes, you're going to get at least 25% more passes in the same amount of time, and that's huge when there are passes that "sell out" by early afternoon...
But the BIGGEST boon from my standpoint is the ability to pull those cancellations late in the day. For example, this past Saturday, around 8pm the standby wait for Space Mountain was 110 minutes or so with no fast passes remaining for the day. Standing in the FP line for Big Thunder with my family (which I also pulled as a cancellation while in the fp queue for Haunted Mansion), I refreshed my app and pulled a fast pass cancellation at Space Mountain for 8:10pm. So after Big Thunder we went on Space Mountain, and while in the FP line there I refreshed my app over and over and pulled another fast pass cancellation at Space Mountain for 8:20pm. So we went on Space Mountain again.
Without Maxpass, those evening hi-jinks wouldn't have been possible. Keep in mind also that I was pulling for a party of 6 - I don't know exactly how the cancellations work, but I imagine a solo person will have more opportunities at cancellations.
You don't need it. But if you like riding rides you're going to want it.