I guess it depends but you really don't want to be paying 18 million a year for any bench player unless they are basically playing the Manu/Harden 6th man role where they are still likely finishing the game and playing starters minutes. His deal only being 3 years would help if it ended up happening though so it's not a total value sink.
Barnes was brought in to be a starter, will be a starter, and play starter minutes. Moreover, unless Sasha proves otherwise, no one deserves to be a starter in his place as of now.
Of course, it can be argued that someone like Kessler starting is more a question of fit vs worth, and Barnes is more valuable with the second unit. That's the argument anyways in the discussion. However, Barnes' minutes can be staggered to give him more time with the second unit. With Barnes, Keegan, and Sasha likely to play majority of minutes at the 3-4 positions, one or both of them will see reasonable time with the second unit anyway.
Minutes distribution will be quite interesting. We have depth at virtually every position (though I would like to see a third PG, unless Keon is expected to fill that role in a pinch. I know Monk will help, but I would like a regular PG). Given that Davion and Monk will be the primary backups at 1 and 2, if Duarte also sees some time, Kessler will likely see most of his minutes at the 3 position. Some of these guys will see minutes hard to come by. Might be a similar story for Len/Noel with Lyles probably being the primary Domas backup. That said, if some of these guys can spell longer rests for the starters, good for everyone.