This advice, of course, depends on the size of your league. If you’re in a 14-team NL only league, you are almost certainly going to have some middle relievers on your team. However, in most 10- or 12-team mixed leagues, you generally are best off avoiding middle relievers altogether, with two exceptions: 1) you expect that they will become closer soon, and 2) it’s late in the season and you can gain ground in ERA or WHIP. Other than that, however, middle relievers should be avoided.
Why? Because they really don’t add much to your team. If a middle reliever gets a win or save, it’s usually a fluke – it’s almost impossible to predict how many wins a guy like Rafael Betancourt will have in 2009, for example. And even the best middle relievers usually only pitch 60-70 innings. So even if you stumble onto a fantastic season by someone like Betancourt, his impact upon your team is minimal, because he’s only pitching 60-70 innings. It’s a very rare middle reliever who is worth having on your team – someone like Mariano Rivera in 1996, or Octavio Dotel in 2001 or 2002. These pitchers are few and far between, and “typical” middle relievers just don’t help you enough to justify a roster spot.
Again, it’s not that someone like Betancourt or Scot Shields isn’t a good pitcher; rather, it’s that their roster spot can be better spent on someone else. In many leagues, you can manipulate matchups so that the roster spot is occupied by a rotating assortment of waiver wire starters who have favorable matchups (in pitchers’ parks and/or against bad offenses). These pitchers may not be particularly good, but if you manipulate their matchups they can provide a heck of a lot more value than even a very good middle reliever.
As mentioned above, middle relievers are generally acceptable when you have reason to believe that they will become the closer very soon. They are also acceptable down the stretch run (generally August and September only), if your team is in the position where you stand to gain points from even a small improvement in ERA or WHIP. In this situation, the small amount of innings that a middle reliever will provide is particularly beneficial. However, in all other situations, middle relievers are almost always a waste of a roster spot.