Is Flying AGL Legal?

I am a member of two local soaring organizations and work with several
others. Most organizations fly MSL. A few teach flying AGL. It was
pointed out at one meeting that flying MSL is illegal if you read FAR
91.121 (shown below). They also mentioned that if you crash and the
NTSB says your altimeter was set wrong, then your insurance (life
and/or hull) may not pay up.

At least one CFIG at the club that teaches AGL says that gliders do not
fly "cruising altitude or flight level" and thus this FAR does not
apply.

What is your opinion? I offer none of my own at this point.

Thanks, John

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FAR Sec. 91.121

Altimeter settings.

(a) Each person operating an aircraft shall maintain the cruising
altitude or flight level of that aircraft, as the case may be, by
reference to an altimeter that is set, when operating--
(1) Below 18,000 feet MSL, to--
(i) The current reported altimeter setting of a station along the route
and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph
(a)(1)(i) of this section, the current reported altimeter setting of an
appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the
elevation of the departure airport or an appropriate altimeter setting
available before departure; or
(2) At or above 18,000 feet MSL, to 29.92'' Hg.
(b) The lowest usable flight level is determined by the atmospheric
pressure in the area of operation as shown in the following table:

Current altimeter setting Lowest usable flight level
29.92 (or higher) 180
29.91 through 29.42 185
29.41 through 28.92 190
28.91 through 28.42 195
28.41 through 27.92 200
27.91 through 27.42 205
27.41 through 26.92 210

(c) To convert minimum altitude prescribed under Secs. 91.119 and
91.177 to the minimum flight level, the pilot shall take the flight
level equivalent of the minimum altitude in feet and add the
appropriate number of feet specified below, according to the current
reported altimeter setting:

Current altimeter setting Adjustment factor
29.92 (or higher) None
29.91 through 29.42 500
29.41 through 28.92 1,000
28.91 through 28.42 1,500
28.41 through 27.92 2,000
27.91 through 27.42 2,500
27.41 through 26.92 3,000