Pro - The person has a boat load of Disney knowledge and knows how to properly plan and not make the rookie mistakes of a first time visitor like arriving to the parks in the afternoon when its crowded and the lines are long, not knowing FP+, ADRs and going during the extremely busy times of year.
Con - Ride selection and in park activities are subjective. What a planner may think is a must-do, another may find lame (i.e. Soarin', Peter Pan's Flight, parades or even character meet and greets.) and vice versa. Restaurants are also subjective. The pro may not understand the preferences of cuisines for the rest of the party or may book a character meal for the "experience" when a person in the party wants nothing to do with characters.
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"All my life I wonder how it feels to pass a day, not above them but part of them.
And out there living in the sun, give me one day out there, all I ask is one to hold forever. Out there where they all live unaware, what I'd give, what I'd dare, just to live one day out there."