We use ACARS for a number of different types of messages, a lot of them are technical and are designed to be deciphered by a computer program.
The messages sent to Flight Dispatch are usually plain text, they may appear to be in code, but at least in our case this probably has more to do with the captains lack of English.
There is nothing secret about ACARS messages, they can be monitored just as easily as a VHF or HF frequency.