Define good gliding weather?
Even if there is no wind, wave or thermal activity, winching still gets you up to 1400-1800ft which gives enough scope for training, all said and done even though you CAN fly above the site why not simply navigate round it as it reduces the percentages.
Apart from that, imagine the AAIB report, Pilot X chose to route over a known gliding site where the mid-air occurred. It's like drinking a pint then driving, not illegal but when the report says the driver had been drinking but was under the limit, you're damned.