It was also being used in the flashback episode where M’Benga and Chapel were medics on the front lines of the Klingon war, they were using an evacuation transporter to store critically wounded soldiers who couldn’t be patched up with the equipment they had on hand. Led to a difficult moral dilemma where they needed to clear the buffer to accept more incoming wounded in need of treatment.

The more routine it gets in the show, the harder it is to explain its absence.