In that sense, is demonstrating
proficiency in it any less "practical" a test item than some of the
ground reference maneuvers found in the airplane PTS?

I did a full slip and got really low at one point, and
my CFI at the time remarked he wasn't comfortable that
low with a wing down.

I took a look out the side and said to myself
"Self, where did those hella long wings come from?"
and then pulled out the slip.

Heck yeah, slips in a glider are a lot different than
in a 152 :PPP