I would say the same thing about designated hitters, to be honest.
You're going to see plenty of more relievers getting into the HOF, since the role of the relief pitcher has gotten so much more important as the dominance of starters has declined.
Mussina really surprises me in this group. He put up good numbers over a long career, but I don't think he was ever considered a dominant pitcher -- let alone one of the best in the major leagues -- at any time in his career.
Unfortunately, the induction of Harold Baines into the HOF is an acknowledgement by the voters that 20% of the players who were in the big leagues for at least 10 years could conceivably end up in the Hall of Fame.
In recent years it seems too many players have been voted into the Hall of Fame only because they reached a certain benchmark like 500 home runs. An example of this is Harmon Killebrew. He was an asset to his team and helped them win a World Series but he had a .256 life time batting average in 22 seasons. Killebrew’s team mate, Tony Oliva, is not in the Hall of Fame despite a .304 lifetime batting average in 15 seasons. Oliva was also a 3-time AL Batting Champion and one of those batting titles was for the 1965 season when the Twins won the World Series. Oliva was also an 8-time All Star. But he didn't reach the 2000 hit benchmark so maybe that's why he's not in the Hall of Fame.