Some folks here have dumped the bale in and never had problems, others had done it once and ended up with a horse with chronic cough and/or heaves.
Throwing the bale in just like that will result in up to 50% waste, so some type of feeder would be necessary
Letting horses eat however they want can expose them to bad, moldy hay which might be hiding in a bale.
There are slowfeeder haynets available now, which reduce waste, slow horses down, which is more natural and better for them.
Throwing it in will reduce waste, and gives you some control over what they eat(you would discard the bad hay).
This is what I did for many years, and it worked just fine.
Now I have square bales only and use small slowfeeder nets. Horses are happy, I'm happy and I save lots of money.