Roll your own? Care to explain? Apologies if I misunderstand but your question sounds like "why bother with email when there is GMail".
XMPP is an open, extensible (hence the "X"), and battle-tested (literally!) protocol that is implemented in hundreds if not thousands of solutions (starting with Whatsapp itself).
Whatsapp, Facebook, various game developers, and Apple (and Google and many ISPs back in the day) managed to successfully run XMPP based solutions for millions of users. This guy's "roll your own" messaging service doesn't seem to have been quite as resilient.
I won't mention the advantages of there being an actual specification for the protocol and a process for getting extensions into and out of it, etc.
Biting the hand that feeds IT
