It depends, both the slip clutch and the shear bolts have their place, both have merits depending on application.
The clutch must be tightened as to provide enough "slip" and PTO rotation to the implement, if the clutch is too tight, PTO damage can still occur.
Conversely, the shear bolt is somewhat idiot proof (something I need) stick the bolt in and tada there you go. The problem exists in the shear strength of the bolt. Too strong a bolt and PTO damage can occur, too weak a bolt and you will be shearing off bolts all the time.
It just depends on the application