Full-time DHs like David Ortiz are very rare these days.
Full-time DHs like David Ortiz are very rare these days. (USATSI)

It's no secret that offense has declined around baseball these last few seasons. The average team scored only 4.07 runs per game in 2014. Five years ago it was 4.61 runs per game and 10 years ago it was 4.81 runs per game. That's a big drop.

The decline in offense can be attributed to many things. Improved performance-enhancing drug testing is just part of it. Infield shifts are widespread, scouting reports are better, pitchers throw harder than ever before, all of that has reduced the number of runs around the game.

As the level of offense in baseball has fallen, so has production from the DH spot. In fact, DH production has dropped at a greater rate than offense in general. Check out this graph from my ruggedly handsome pal Michael Barr of Lookout Landing:

wOBA, or weighted on-base average, is a souped-up version of on-base percentage. It's the same general idea -- wOBA does weigh singles differently than doubles, etc., unlike regular ol' OBP -- and it runs on the same scale. A .360 wOBA is just as good as a .360 OBP.

The average DH wOBA (blue line) has a steeper slope than the average wOBA for all positions (orange line) over the last 20 seasons. The gap was approximately 18 points of wOBA in 1995, but in 2014 the gap was down to 10 points. DHs are still above-average hitters overall, just not as much as they once were.

The full-time DH is a dying breed. Guys like David Ortiz and Edgar Martinez are very rare. These days teams will rotate players in and out of the DH spot and use it as a way to rest players once or twice a week without taking them out of the lineup. It's good idea in theory, but it hurts offense because being a DH is hard.

There has been lots of research showing pinch-hitting and DHing hurts a player's offense, somewhere in the neighborhood of 5-10 percent. It's a tough job, sitting around on the bench between at-bats. A lot of players struggle to find ways to stay sharp and focused in those roles, hence the 5-10 percent penalty.

Even though he doesn't play the field, Ortiz is very valuable because he's mastered the art of remaining productive as a full-time DH. Billy Butler has done that as well, 2014 notwithstanding. Even though it limits flexibility, teams could stand to gain some offense by finding someone who can DH full-time, and these days anything that helps score runs is a major advantage.