( wonder my my MFG and the others that posted recommend shutting off the drop speed when in transport )
They are worried about safety.. not specifically equipment damage.
Like I said.. do whatever your manuf. reccomends.. but... I however just got done rebuilding the hyds on a tractor that took off an on all weekend due to a blown out gasket from a shock load to the 3pt.
Also.. like the other posters posted above.. the 3pt lockout is not a failsafe device.. and I was only pointing out that it can fail, as it is a valve restrictor.. not some sort of mechanical 'block' that physically prevented the lift from dropping. Also.. if your lift cylinder leaks.. the lockout valve will do little to keep the lift in place.
Soundguy