I believe SIPRNET has been around for many years and is very secure. The military has used chatting software and other communcations software for a long time. So, using mIRC or some other chatting program isn't a real issue. Also, if they use a decent encrypting script (or, for that matter, use external software to encrypt and decrypt the text), just logging the information won't help any more than listening in on their audio communcations in the field. Encryption and codes change all the time, so even if you crack it once, it won't let you keep it cracked forever without recracking it all the time.
As long as they aren't flying the planes with mIRC, I don't see a real problem with it. Besides, they could easily script in protections from users typing things like $decode.
