#2 They won't have to run the same match-ups
While the talent of The New Day and The Usos is apparent and useful, there are only so many times that we need to see them fight each other. A new team might get added into the feud to bring a new wrinkle to the pairing, as The Miz and John Morrison were on the build to WrestleMania 36.
When The Revival was still a part of WWE, they were often feuding with one or both of the teams. The action always delivered but it kept other teams from getting a chance. Too much of the same thing can become stagnant.
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The same hasn't happened a lot of the main roster yet but the Street Profits and the Viking Raiders fought in NXT a few times. Erik and Ivar bested Ford and Dawkins each time and it was woven into their upcoming match on RAW next week. When we constantly see the same two or three teams always battling for the titles, it only usually works if there is a drastic change in the dynamics of the feud (heel/face dynamics).
By having only one set of tag team belts, it would allow different teams that are worthy to get a chance. It would also prolong the potential pairings of teams that always face each other by offering up a break from constantly facing each other.