I don't think you can underrate having some scoring off the bench. You need to have guys that come in and score if the starters are having an off night. Kyrie and LeBron have both had poor performances in the postseason so far.
Also, I'd cite Boston as an exception, not the norm. Boston (probably more so than any other team in the league) relies on their bench scoring as the main component to their offense, and not just a supplementary one. Their best player comes off the bench, and their starting lineup is average at best in terms of talent.
I do get what you're saying. This team, as constructed, gets the majority of its scoring output from its starters. You're not going to win many games getting 20 or less a game off your bench. There's no need for their bench to score 40-50 a game, sure, but they do need to have reliable guys that can come in and score while the starters get some rest. Besides, Smith is in at the end of the games, anyway. Does it really matter if he's not starting as long as he's closing out games?