(maybe all - I don't know of an exception) flight manuals
tell you to
never land gear down.

Uhh, gear up you mean? : ) There are exceptions however,
like in a water landing for instance. You don't want
the gear down as it causes the glider to violently
pitch down and submarine on touch...err-splashdown,
greatly increasing the risk of smashing into the lake/river/ocean
bottom and/or drowning.
Uncle Hank where are you?
Another exception is my Sisu 1a. It has a shock absorbing
oak skid with a steel shoe, in addition to a retractable
main. If the field looks to cloddy or the grass too
tall it is recommended not to lower the gear. AJ Smith
gave it it's first wheel up (unintentional however)
and Johnson the second, both without damage to them
or the ship. But landing gear up is generally a bad
habit, for many reasons, including having a crumple
zone.

Paul Hanson
"Do the usual, unusually well"--Len Niemi