My understanding of crush syndrome is that the longer someone is crushed, the more toxic substances accumulate.
Relieving crushing will release this 'trash' into the circulation but there'll be less if pressure is relieved sooner. With any luck, the emergency services can get a liberated casualty to hospital sooner. Despite the risks, the emergency services will have to free the casualty anyway, though they will be best equipped to deal with any drop in blood pressure or bleeding that may occur.
All in all, 'having a go' and helping free a casualty is likely to be more beneficial than doing nothing.
But it's a fine balance.