Let's start with this qualification: FHIR and Blue Button Plus are related but not identical initiatives. The former is an HL7 International standards effort. The latter is a US initiative that is using parts of FHIR to enable consumer access to clinical data.
What Blue Button Plus (pull) provides is access to a FHIR-based resource by which you can query for CCDA documents in a RESTful manner, and subsequently download them. It doesn't ask you to aggregate your content differently, or use some new format to describe problems, medications and allergies. So, content is structured the same, but metadata is structured and queried RESTfully.
The mechanism by which access is granted via BB+ Pull makes use of OAuth 2.0, which is the same thing that FaceBook, Twitter, Linked In, and other services on the web enable other applications to access your data stored in those services.
Why is BlueButtonPlus / FHIR good?
This is good because it vastly simplifies the way that consumer applications can access health data, making it possible for smart phones, tablets, wearable devices, et cetera, to participate in consumer data access and use of healthcare data. It also prepares sources of consumer data to move towards an architecture that can support more granular access in the future (but doesn't require it today).
Does Blue Button Plus / FHIR eliminate the need for CCDA?
Blue Button Plus is not eliminating CCDA, but rather building from the use of it. Blue Button Plus REQUIRES the use of CCDA.
You've probably heard about "Green CDA" and I've said in the past that FHIR is to HL7 V3 what Green CDA is to CDA. FHIR greens HL7 Version 3, and so EVENTUALLY, there will be a new Document resource in FHIR that is a composite of clinical resources (like problems, allergies, medications, lab results, et cetera). But we (HL7 Structured Documents) aren't working on that right now. Eventually this will occur, and those resources will be key components of it.
How is Blue Button Plus / FHIR retaining compatibility with CCDA?
CCDA and Green CDA and content from similar efforts are being used to help define FHIR resources. The document resource enables access to "FHIR-based" documents as well as those that existed prior to the development of FHIR (like CCD or CCDA documents). Present efforts in Blue Button Plus focus on the latter.