The Vanishing 100-Inning Reliever

Tyler Clippard leads the major leagues in innings by a reliever with 25; he’s on a pace to throw 115.2 innings this year, all in relief. Manuel Corpas is #2, and on pace for 111. In the AL, nobody is on pace to crack 100 innings solely in relief – Joel Zumaya is on pace for 98.2 innings in relief.
With deeper bullpens, even in the face of declining innings by starters, the 100-inning reliever has become an ever-rarer species. Looking decade-by-decade just at guys who cracked 100 innings without starting a single game (thus skipping over the guys who pass 100 relief innings plus a few starts), we see the rise and fall of the 100-inning reliever (and why Mike Marshall will almost certainly remain the only man to pass 200 innings in relief in a season):
1930s: 3
1940s: 8
1950s: 15
1960s: 67
1970s: 100
1980s: 115
1990s: 29
2000s: 6
The first guy to do it was Clint Brown in 1937, the last Scott Proctor in 2006 (what’s with guys named Scott? The last before him was Scot Shields, and the last to do it more than once was Scott Sullivan in 1999, 2000 & 2001), so we’ve already passed three straight seasons without a 100-inning reliever. And the guys on pace in mid-May to just clear 100 are usually not great bets to keep that up all year.
As with many pitcher-usage issues, there are good reasons why innings have been declining (see my history of pitcher workloads), but no particular reason to think that managers are currently striking the right balance between avoiding injury risks and handing too many innings to second- and third-tier pitchers. Mariano Rivera and Derek Lowe both survived 100-inning relief seasons without doing any great damage to their arms. But the game continues to move in that direction regardless of whether anybody is analyzing whether it makes sense.