For some its about education - educating others on their own belief, or in a backhanded way getting some education on another's believe, by possibly pushing them for the answers.
It has been said that if your faith/religion cannot stand up to scrutiny, should you not also question it? (Paraphrased..I forget the actual saying) - Also, does not questioning of your faith, by another, help to strengthen your position?
The abuse is unnecessary, and I for one am tired of the generalisation by certain people about different groups - eg: that atheists HATE religion, or are evil, or have no morals, or just don't want to believe so they can sin (all wrong) (seen this from Christians, and Muslims and others), or that ALL Muslims are violent, evil, etc etc (many times I've seen this from quite a number of Christians and several Atheists), that all Christians are ignorant, hateful (several atheists...which saddens me).
Generalising a group based on it's WORST members is as ignorant as ignorant gets. Particularly, if the person doing said generalising is upset by the same sort of generalising being done about their own religion.
What is the point? I have no idea. I ask questions, like my recent one on violence in religion - to either answer a question I personally have, or hopefully get people to think before they generalise the next time.
Abusiveness however, is unproductive, and childish. So I try to avoid these people.