Without the play-ins, many teams would've rested their main players with minor injuries to heal and give way to the other players.
I guess that statement can only applied in the Eastern Conference where as always, you can see there's not much close competition for the bottom playoff spot. Before the involvement of the Play-in tournament, the Eastern Conference is always like that. Even though there are still several games remaining in the season, if the team doesn't have a chance now to enter the playoffs, key players are now sitting out or just having a limited time of playing.
Different from the Western Conference every season, you can see teams are battling closely with each other and even those teams from 9th spot to 12 spot can still change the final playoffs lineup. And become more interesting and thrilling because of the addition of a play-in tournament.