The poison is dangerous to some people because roaches carry a lot of nasty bacteria, so ingesting the poison can lead to life-threatening food poisoning.
It's not only the roach that's affected by toxic bait; any man, woman, or child who eats anything containing roach toxin will be poisoned and may die. For this reason alone it should be avoided at all costs.
When using roach poison, take care to avoid coming into contact with your eyes and mouth. Most insecticides that are safe for most people may not be safe if you have a chronic exposure - i.e., use them every day or at high doses over a long period of time.