What Joe said... but lets not use Adam Dunn as the example.. lets use Ortiz.

When bosox play interleague game at home they get to play Ortiz, NL team plays its best utility bat (still not as good as a DH in most cases).

Sure in the NL parks the AL has to bat a pitcher that normally doesnt bat... but what are you really giving up from a pitchers at bat compared to what the AL team is gaining when having the NL team visit.