PGP is needed, because it does something few applications do -- it works regardless of the transport layer. I can PGP encrypt a document, E-mail it, or I can send it via SMS, MMS, copy it to a SD card and put it in a dead drop, post to alt.anonymous.messages, or any number of ways. In any case, the document will be encrypted, and signed, so the receiver is assured of its security, no matter how public the transportation is.
Of course, PGP isn't perfect... it is a standard made in 1993. It needs forward secrecy, and a better binary to ASCII encoding scheme, preferably with the option of adding ECC. But it does work, and it is secure. It also will work with whatever the latest trendy messaging app will be come next year, just as well as it will work with ones from the past.