I would say the closer role is overrated because it has become a ninth inning specialist. It's nothing about Kimbrel, who is a terrific pitcher. But the way this role has evolved has made it into an overpriced specialisy. (It is odd how baseball economics have evolved from using specialty players in order to save money into watching them get overpaid for their specialization.)
The game is simply not on the line in every ninth inning. Plenty of times, far more critical situaions happen earlier that that. We've all seen too many games when other bullpen pitchers come in to face the 4-5-6 hitters with men on base in a close game in the eighth inning only to see the closer than get rolled out in the ninth to face the 7-8-9 hitters. To me, this is a waste of having a pitcher like Kimbrel.
Kimbrel pitched 61 IP this year, but 53.1 of them were in the ninth inning. I'm not going to go through every game log, but if there is an even distribution, then roughly 44% of those games, he started the inning off with a hitter batting 6 through 9. These are the easier outs in every lineup. If closers were still used like they were back in the days og Gossage, Sutter and Fingers, that would be different. Those guys were not ninth inning specialists, and often pitched 2 or 3 innings in many appearances. They rarely if ever had games where they faced the bottom of the order and no one else.
Also, as most managers manage towards to save, the closer is used for about any lead of three runs or less. Using your best reliever with a one run lead certainly seems wise. But with a 3-run lead? Sometimes that is overkill, espeially for those 3-run leads against the bottom of the order. I really see no point in saving the best reliever to face the 7-8-9 hitters with the task of getting 3 outs before giving up 3 runs.
I'd rather see Kimbrel (or any closer) used when the game is actually on the line. This does mean coming in with men on base in the seventh or eigthth innings. While you might not like the idea of using Kelly in the ninth, why do you find the idea of Kelly facing the heart of the order with men on base in the eighth inning acceptable? Or Hembree or Barnes?
The problem with using the closer when the game is actually on the line is many pitchers and their agents on't like it. Saves have become a bargaining tool for agents and being a closer does mean more years and money when negotiation time comes. Remember when the Andrew Miller contract ($8mill per for a middle reliever) sent shock waves through the industry? That was closer money at the time. While Miller was typically facing those tough pre-ninth situations, most teams did not want to see non-closing relievers start to get paid like closers. And after that deal, we alsos saw a severe rise in what closers were paid, despite not always handling to toughest situations.
The role of closer as it is used today is certainly overrated. I'd rather have a pitcher who can put out the tough fires than an overpaid ninth inning specialist. But it is very tough to find an Andrew Miller type who wants to take on that role, and the reason always boils down to financial ones.
And if I asked you hypothetically, how much worse would the Sox be if, say, Kelly was closing, what would you answer? Certainly the Sox wouldn't still be fighting with the Yankees for the division. Maybe home field would still be in doubt. Maybe. I think that was the original point. However many games Kelly would have cost the Sox by being a ninth inning specialist, that number is probably less than the number he did cost the Sox by pitching in crucial situations in the seventh and eighth innings.
I like the idea of having a pitcher as good as Kimbrel in the bullpen, but if he is only going to pitch the ninth inning regardless of the situation, it is overkill. Because saving your best reliever to protect a 3 run lead for one inning against the 7-8-9 hitters is not a good way to use him...