Why Does Flashbuilder 4.7 need 2 versions of the Air SDK?

It looks like we need 2 versions :

(a) C:\Program Files (x86)\Adobe\Adobe Flash Builder 4.7\eclipse\plugins\  -  Has sdk with new compiler

(b ) C:\Program Files (x86)\Adobe\Adobe Flash Builder 4.7\SDks\4.X.0        - needs overlaying with SDK without Compiler

Can someone explain why both are needed now that Flex is not supported?

Related to that if FlashBuilder is not being used (E.g. doing command and line builds via mxmlc) which SDK

should be used?

Thanks