I'd certainly call it dangerous.
It could be you reaching back there, and you remember it is exposed and are careful.
But it also could be your spouse, trying to unplug a misbehaving machine.
Or an appliance installer or repair person, with no knowledge of the missing cover.
Or it could be one of your kids, after you're gone, and nobody is there to warn them.
It's 240V, it's open, and it's on the back where it can't be seen until your hands have been back there. Just put a cover on it.