At least at Nike the athletes/celebrities providing input had a huge amount of experience in that field to know what the performance issues were, what they wanted improved and that in turn would influence their play, making it extremely valuable advice to apply to the design process.
Even if they had stopped performing to help full-time to work on product design, do they use technology so differently that their input is more valuable than what regular consumers provides? Basketball shoes I understand absolutely how pro input influences shoe usability and the brand itself. A phone…not so much.
Would I buy some basketball shoes because Michael Jordan helped design them? Sure, he backs up his input with experience.
Would I buy a phone because Alicia Keys had some kind of input? Nope.