I think the biggest reason for many modern chat services to disregard XMPP is, because they fear that most advanced users might resort to their Adiums & Pidgins and wont use their web-services or Apps at all, resulting in half of the customers not experiencing the UX or unique features and therefore the value.
We had a long discussion in the beginning of the project and decided to make our architecture XMPP-friendly, but release it later in the future, as we need our early adopters to use the system.
So the answer to XMPP is "Yes, but after we gathered enough feedback"