[16NG] question about RFC4840

Hi! all,

I have a prolem of rfc4840.

In section 2.2 multicast/broadcast: 

   In wireless media where data rates to specific destinations are
   negotiated and may vary over a wide range, it may be more efficient
   to send multiple frames via link-layer unicast than to send a single
   multicast/broadcast frame.  For example, in [IEEE-802.11.2003]
   multicast/broadcast traffic from the client station (STA) to the
   Access Point (AP) is sent via link-layer unicast.

It is because it usually is physical "unicast" from the STA to the AP.
It is the point to point link. 

As there is no words about the link from AP to STA,
the following recommendation seems to be problematic.   

   Recommendation: Where support for link-layer multicast/broadcast is
   problematic, limit the propagation of link-scope multicast and
   broadcast traffic by assignment of separate prefixes to hosts.  In
   some circumstances, it may be more efficient to distribute
   multicast/broadcast traffic as multiple link-layer unicast frames.

In many circumstances, it is less efficient to distribute multicast/broadcast 
traffic from AP to STA in multiple unicast frames.

I want to know what are the circumstances that the author wrote. 

Regards,

Gengyu