A lot of these borderline cases Spurs, Bucks, Cavs it is going to come down to coaching preference that we are just guessing at.
For example, on the Cavs nobody could convince me that they wouldn't be a better starting 5 with Smart over Smith. The Cavs were absolutely horrendus on defense for most of the season and even worse the second half of the year. Smith is clearly on the decline as a 32 year old that relied heavily on athleticism. Whether it age or rust, he is just not shooting the ball well anymore and continues to make bone headed plays (he is the opposite of smart in this way)
While I could buy the argument that there is some ego and contract decisions that would complicate it, I think Lebron would probably love Smart and would say they needed his defense on the first unit (Kyrie is awful) and Smith would shut up cause he knows where his bread is buttered.
While it is an interesting counter point regarding shumpert the Cavs coaching staff just doesn't love him. For a team desperately needing defensive stops he is only playing about 15 minutes a game in the playoffs. Either his defense his slipped or there is something else going on on a team really needing defensive players.
I think the Jazz would go Hill, Smart, Hayward, Johnson, Gobert and have a really nice mix of shooting, rebounding and offense. I actually think that team would be a bit scary for teams to play.
I think taking all these cases, you can at least make a reasonable argument for Smart starting for half the teams in the playoffs with 6 of them probably being pretty clear cut.