I'm inclined to put the case for spinning gliders very, very simply. Gliders spend a lot of time turning close to the stall (aka thermalling), therefore, whilst they are spin resistant, there is far more likelyhood of entering a spin when gliding. If you carry extra speed as a margin, you simply don't climb as well.
BackPacker - the '21 is the most spin resistant thing I've ever flown - we don't spin them as they're reckoned to be unspinnable without tail-weights (for a more rearward CG), and yes, that's a factory thing, not simply someone's neat idea(!) There are other gliders that spin a whole lot more convincingly! Spinning gliders is also a wholly different ball game to powered, both in height loss, and violence.
To Mary's question, UK glider teaching is (or was, when I trained), firstly, think before you do it, and don't put yourself in a corner, BUT if in trouble/doubt PULL. The reasoning being that during certification VNE is set at 90% of the max demonstrated speed, whereas the structure has to demonstrate 150% of the max G load (usually 6G) and remain flyable. Doubly important as gliders tend to be very slippery and gain speed rapidly, and I suspect more flutter prone with long wings. Also, heaving a lot of 'G' tends to slow things up pretty quickly, leaving you unable to pull lots of G very soon.
We are also taught to recognise the difference between a spiral and a spin. One of the key ones being that a spiral shows rapidly increasing airspeed, whereas a spin is usually either unreliable (too much flow across the pitot), or steady.
And finally, to Abbey Road's point, pulling too hard (stalling) in a turn *should* not result in a flick, if the aircraft is in balance - being flown correctly - which is back to Pace's point about driving
(ok, I'm being a bit flippant there..)