Never heard of a need to break one. They have opening on the bottom you can open, or the slit is big enough for the coins to fall out when you turn it upside-down and shake.
Why would anybody ever use them? It's a safety hazard.
Why would one not use them? It's fun to toss coins after you do a "bad thing" like swearing around children.
Also, what safety hazard? From whom? Family? They can wound you much worse with words alone than breaking some porcelain piggy.
And if you mean being hurt on potential shards... well... You have a greater chance of choking yourself to death using your own saliva than being wounded by smol piggy, even if broken, as its fragments aren't sharp enough to break your skin unless you really try.