Prince
1,487 minutes / 942
Sprocket points 6.3 per ten minutes
Hayes
718 minutes / 361
Sprocket points 5.0 per ten minutes
Herrmann (including Charlotte)
248 minutes / 134 Sprocket points
5.4 per ten minutes
Pietrus
641 minutes / 356 Sprocket points
5.6 per ten minutes
Both Herrmann and Pietrus are upgrades over Hayes right now overall. Hayes shoots slightly better than Pietrus, but is less involved in rebounding, assists, steals, blocks, etc.
It seems the Pistons want a backup SF who provides scoring punch off the bench. For that, and only that, Hayes is a better choice.
FGAs per ten minutes and points per ten minutes...
Prince - 3.4 / 3.9
Hayes - 3.8 / 4.0
Herrmann - 3.4 / 3.5
Pietrus - 2.9 / 3.5
Still not exactly sure why I wouldn't rather have Herrmann.