The question isn't really the minimal bandwidth of the PHY rate it's about the goodput for a given reliability. Regardless of your radio there will always be some point where someone is at the edge of a connection and goodput is less than minimal PHY bandwidth. The call then turns choppy/into a time stretched robot you get every other syllable from. The less data you need to transmit + the more FEC you can fit in the goodput then the better that situation becomes.

Not to mention "just because I have some minimal baseline of $x kbps doesn't mean I want $y to use all of it the entire time I'm on a call if it doesn't have to".