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Good discussion here. Here's a few additional thoughts/comments:

First, multicast must be sent at a Basic Rate, but not necessarily the lowest:
From 802.11-2007, Section 9.6,
"All frames with multicast and broadcast in the Address 1 field that have a UP of zero shall be transmitted at one of the rates included in the BSSBasicRateSet parameter, regardless of their type or subtype."

Second, multicast also has the problem of non-priority. In other words, all group addressed traffic is assigned to AC_BE, so for voice or video streams, you don't get the priority you need.

Third, multicast is not acknowledged in 802.11, which may lead to high loss.