Griffey belongs there. Follow this link and you’ll see, in the first table, that he’s number 45 on the list of offensive players whom I consider deserving of the honor.
Piazza doesn’t belong there. He falls short of the 8,000 plate appearances (or more) that I would require to prove excellence over a sustained span. Piazza would be a true Hall of Famer if I relaxed the standard to 7,500 plate appearances, but what’s the point of having standards if they can be relaxed just to reward popularity (or mediocrity)?