Really, what you have is two forces working together.
1. The 9th inning is not special. Leverage is a little higher - but fundamentally, the job of getting 3 outs without allowing a run entering a game with the bases empty is not especially difficult.
2. Workers like to understand their job function. Pitching in the 9th is a job - not the most difficult on the staff, but a distinct one. It is hard to shuffle roles and I do think that the bullpen jobs are not 100% substitutable.
So - essentially having a "closer" is critical. But that closer does not have to be your best reliever. You look at the long, long, long line of mediocre pitchers who have done that job adequately (Mike Williams, Doug Jones, Heathcliff Slocumb, Joel Hanrahan) and that should tell you something.
It is one of the interesting pieces of the Kimbrel acquisition that will be tricky. Neither Kimbrel nor Uehara are guys who you feel comfortable giving larger, more variable workloads. They both should be very effective - but they both will probably need specific targeted work. Farrell will have to use Tazawa and Smith to help organize the game to facilitate things for both of them. It is a little harder than the normal bullpen juggling. Fortunately both pitchers are very capable.