

There are several reasons for this difference.  The main reason is that
good hitters in their prime (such as Frank Thomas and Danny Tartabull)
are rarely regular DH's, even if they can't field; it's more commonly a
place to put aging veterans.

Another use of the DH slot is to help a player come back from injury
slowly, getting his bat in the lineup without forcing him to play in the
field.  I would expect such players to hit worse as DH's than they did
during the rest of the season.

A third factor is that most player who split time between DH and first
or the outfield don't hit as well as DH's for some reason.  Reggie
Jackson was a good example; in five years as a RF/DH for the Angels, he
hit like the old Reggie when he was in right.

The second and third factors will be lost if players are considered by
their primary position only; a player who plays 90 games in right and 60
at DH will count only in the right fielders' averages.


This makes sense; you might also look at it another way.  Although the
*average* offensive performance of a DH may be lower than for a left
fielder, the *replacement-level* offensive performance is certainly
lower; if you are a left fielder but can't hit well enough to keep a job
anywhere, then you certainly can't hit well enough to keep a job at DH.

