I think they are two situations :
1- The phone owner need to keep secret his data
2- The owner doesn't have sensitive data regarding the laws, he is not willing to take the risk to be jailed or pay a big amount for that.
In first case the ideas discussed here and others treads will adress this issue.
In the second case, "Read only" together with
countdown and no data transfert options would be useful to :
- keep data in the phone
- prevent the data to be shared or leaked too easily
- to some extent protect privacy
- help to prove the suspect innocence
- keep the data immutable