I was taught that there is nothing inherently dangerous in using spoilers in the circuit, provided that the correct speeds are maintained.

With spoilers out, even just cracked open, during base and finals turns safe speed margins are reduced, i.e. inside wing flying slower, reduced lift from deployed spoilers and wind shear (again inside wing flying slower).

However, I was advised NEVER to use them on the base and finals turns (unless absolutely necessary and me paying full attention) and here is why:

Imagine that you normally use spoilers during these turns, with no problems at all, but one day you are returning from a 9 hour cross country flight, it’s been a long final glide and energy is low. Also the weather has changed and the wind is gusting from an approaching storm front.

So you are fatigued, the glider is in a low energy state and wind is gusting. The holes in the cheese line up and you crack the spoilers during a steep base turn with speed too low. The end.