"For example Manchester City had three meaningless, purely decorative stars until very recently, and depending on the model (home, away, alternate, etc.) they have been of various colours and shades."
The Man City one was for traditional and not as you suggest meaningless or to simply keep up with the joneses. Most stuff have reason behind it especially clubs who have history and tradition.
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The current club crest was adopted in 1997, a result of the previous crest being ineligible for registration as a trademark. The badge is based on the Arms of the City of Manchester, and consists of a shield in front of a golden eagle. The shield features a ship on its upper half, representing the Manchester Ship Canal, and three diagonal stripes in the lower half. The bottom of the badge bears the Latin motto Superbia in Proelio, which translates as Pride in Battle. Above the eagle and shield are three stars."