They are usually used for recovery, although they increase strength, it hinders flexability, if they were to use on a constant basis, when they came off, they would be MORE prone to injury.
The hardest I've seen someone throw was that freak (can't remember his name at the moment) from Detroit who regularly throws in the upper 90's to low 100's. Remember pitching power doesn't come from a strong arm, transfer of power ALWAYS starts below the knees.