Most quotes in presentations do not add to the story:

• Too long to read
• Too many buzzwords and generic language
• Given by a person nobody has ever heard of
• Given by a person with a position that is not very impressive (junior analyst at unknown consulting company)
• Give by a person who is very famous but has nothing to do with the subject (Ghandi)
• Used too many times

What can you do better? Find the right person, and get them to say something specific, clear, and simple: "This solution saved the launch of product [x]!" 

UPDATE February 2018: I have added a new post about using quotes in PowerPoint to the blog