Fair point, and one that should be considered on a player-by-player basis. But I'd say that a player who plays a substantial portion of his career as a DH shouldn't be penalized because he didn't play another position. I'm not necessarily saying they should be in the HOF, but I'd give guys like Edgar Martinez (69% of career games at DH), Harold Baines (58%) and Paul Molitor (44%) as full consideration as any player.
I see what you are saying, I just think that their not playing should be taken into account. Choices for the HOF are in esseence the judgement of a players value relative to anothers. Edgar Martinez's value is less compared to a player who has the same batting ability yet was also an asset in the field.