Glider transponder on?

On August 28th, Hawker biz jet and a glider collided south of Reno,
Nevada at 16,000 feet. Fortunately, everyone survived. The Hawker was
IFR and flying a STAR into KRNO. Following is an excerpt from the
NTSB's preliminary report (LAX06FA277A):

"The glider was equipped with a panel mounted communication radio,
global positioning system (GPS) unit, and a transponder; however, the
pilot did not turn on the GPS and transponder. The transponder's
activation is not required for glider operations (for more details see
14 CFR Part 91.215). According to the glider pilot, he did not turn on
the transponder because he was only intending on remaining in the local
glider area, and because he wanted to reserve his batteries for radio
use. The glider was equipped with two batteries (one main and one
spare), however, due to the previous glider flights, the pilot was
unsure of the remaining charge in the battery."

I'd be interested in opinions about the legal conclusion the NTSB makes
in their preliminary report as to whether the glider pilot did, or did
not, have to have his transponder turned on:
"The transponder's activation is not required for glider operations (for
more details see 14 CFR Part 91.215)."

My take on 91.215 is that the NTSB is wrong. It was elective to install
a transponder in the glider, but because it was installed it had to be
on unless it was broken