#2. Not all superstars are cut out for off-script promos
As mentioned, one of the weakest aspects about the WWE product today is that with a renewed focus on the in-ring style and flashiness, the art of promos seem to have gone away. Sure, the top crop of superstars generally are good promos, but if you look at the roster as a whole, not many can cut intense promos and connect with the audience via the mic.
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Perhaps that is more on WWE and how they train superstars at the Performance Center, but if they do feel that superstars aren't capable of cutting promos on-the-fly and improvise like they did back in the day, then it does make sense that they would want them to follow the script verbatim.
Unfortunately, that hampers a superstars' ability to cut promos more, but ultimately, only a few can cut them at an elite level.
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