Take away Smart and the identity of the team will change. This is a team that plays hard more often and more consistently than any other team in the league. Smart is the reason. Once he locks in, his teammates pick up the intensity as well and he locks in during most games.
Granted when you add Haywood, the team is still going to be really, really good, but without Smart, they're also going to drop some games on off nights - games that they probably win with Smart on the court in the 4th quarter.
Plus when you're putting Kyrie, Haywood, Tatum and Horford out there at the end of games - you can afford to have one guy who can't shoot out there with them.
The thing with Smart though is he shoots way too many 3's. He's got to reign that in.