If we are going by existing implementations, there is probably more than one flavor and then there is the question of when the MSK is directly delivered to the authenticator and when it isn't and how the peer knows that.
The Wifi Alliance (WFA) WPA2 certification includes EAP interoperability testing. Quite a few implementations have passed certification, including most of the major ones.
The WFA EAP certification requires that EAP implementations support the generation of the MSK as specified in RFC 2716. Authenticators/APs are required to support transport of the MSK as described in RFC 2548.
Existing EAP lower layers that utilize EAP keying (802.11i, IKEv2, PPP, 802.16e) base their key hierarchies on the MSK. Since the key hierarchy is based on the MSK, if it were not delivered to the authenticator, the exchange of data frames between the peer and authenticator would not be possible.
In this case, I tend to agree with Charles that it is better to have to fix non-compliant implementations than try to design around them.
RFC 2284 was approved in March 1998; the first widely available implementation shipped in December 1999. Major interoperability and security issues were being resolved as late as 2003; testing and certification began in 2005. So it took 7 years to go from initial specification to certified interoperable implementations.
While I would expect that RFC 3748-compliant implementations will take less than 7 years to appear, we are still only at the beginning of the road. For example, there are no RFC 3748-compliant implementations shipping yet; we have not had an RFC 3748 bakeoff demonstrating interoperability between two distinct implementations with respect to new RFC 3748 features (including the EMSK); no open source RFC 3748-compliant implementations exist yet.
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