Last Post: September 21, 2006:
IEEE P802.11-REVma/D5.2 March 2006 section 9.6 Multirate support
"No STA shall transmit a unicast frame at a rate that is not supported by the receiver STA, as reported in any Supported Rates and Extended Supported Rates element in the management frames."
"In the case where the supported rate set of the receiving STA is not known, the transmitting STA shall transmit at a rate selected from the basic rate set or a rate at which the transmitting STA has received a frame from the receiving STA."
I reason that probe requests carry supported rates information to allow the probe responder to more intelligently select a rate with which to reply.
I think an association responder may have to be more demanding of supported rates information from the requester before granting an association. I haven't searched the standard for clues on this detail.
I hope this helps. Thanks. /criss