Just about every theme park does the hand stamps now. At Disney, with the computerized turnstiles, they're almost useless, EXCEPT to make sure you're not sharing your Park Hopper.
People would go to a park, leave, and give their ticket to a friend who would go use it for a while. Back in the day when Cast Members did their jobs and actually checked hand stamps, this was prevented, but like others said, I've only had mine looked at once in my life.
When you put your ticket thru the turnstile at each park you hop to, it knows where you've been, etc. NO need for the stamp unless the computers were to crash, or they think you'd sharing your ticket.
I was also told by a Cast Member that the computer notifies the Attendee if your ticket went through the turnstile twice within too short of a time (like if you passed your ticket out to a friend over the fence or something). Another reason to check your handstamp.
Just in case, I NEVER leave the park without a stamp.