You mean rides that used to have turnstiles "back in the day"? When introducing new technology, its always better to standardize to help with the learning curve. Having easily identified readers with standard markings at park entrance, FP+ lines, and registers helps with adoption.
As a comparison, try using a mobile device at checkout at a retail store. There are so many different types of readers, icons, and procedures that it can drive you crazy. On some machines you can't tell if it excepts mobile payment or where to place your device.
Having easily identified readers and instructing guests to "place Mickey to Mickey" is much better than theming readers which might be difficult to see and saying "place Mickey to the Sourin' icon [or ride specific logo]".