A safety button or switch is used more often on an automatic or assisted knife than on a manual knife. I can see that a button lock might be more prone to being accidentally released than some other types, such as axis or compression type locks. That is why buttons are usually recessed as they are on the Cogent. Still, I think there is little risk from a button lock on a manual knife carried tip-up.
If WE does fell compelled to add a safety, please make sure that the safety itself cannot be accidentally set or released!