I don't know this for sure, but I'm not under the impression that RDRAND has the same "entropy as a scarce resource" or "block waiting on entropy" issue that the Linux apis have.
RDSEED is actually what people want most of the time. RDRAND may return before sufficient entropy has been gathered.
Basically, RDRAND was made first to be something like a hardware /dev/urandom. Of course, people started using RDRAND to generate RSA-keys or whatever, but the entropy may be depleted between uses.
So you're supposed to use RDSEED whenever you need something that'd actually be truly random with a full set of entropy.
