Ah, what would life be without a controversy? Here's some conflicting
information to digest.

Open spoilers are rain catchers. Closed spoilers are not completely
sealed. If it rains, water will dribble in. If it rains hard and long,
the spolier box will slowly fill and at some point begin to spill
through the control rod access hole into the wing.

Solution, leave the spoilers closed and tape the seams. Or better yet,
get a set of wing covers.

BTW, I'm not sure putting the spoilers up wins you any net advantage.
It reduces the lift the wing is producing a little, but at the cost of
more surface area presented to the wind. The only real advantage I see
is that it accuates the wheel brake on many models. The key to
securing an aircraft for weather is no slack in the ropes. I suspect
deploying the spoilers is part placebo, a convenient "extra" for those
who are content to stick a pair of stands under the wings for the
night and call the glider secure.

Those contributors mentioning birds, beasts, and bugs also have a good
point. Taken in whole, I'd vote to leave 'em closed.