I use tiers. So I have 2 As and 2 Bs.
I'll use a closer if and only if he is pretty strong, but has a low DUR/STA combo and will get less than 45-50 ip even if I used him as a Set Up A.
That's the real reason a lot of owners don't use the closer role. A great RP who could log 65-75 ip (or more) at setup A, will only get 45-50ish innings as a closer.
So they want to maximize the amount of innings their best relief guy will get by having him pitch more.
Some still use a closer and argue that "all innings aren't the same" so they'd rather a guy ONLY pitch in 9th inning high leverage situations. I disagree with that because there are many other high leverage situations, and I want the best guy available to get the ball in as many of those as possible, not just in the 9th.