from an Islamic perspective more than anything, what is the use of debating with people of other religions?

i mean i sometimes find myself refraining from asking questions like (for eg) "whats the difference between Christianity and Catholicism...what the difference between a roman catholic and a catholic" because i feel that alot of the time people are going to be replying based on their own opinion since the bible has been distroted, etc and (i dont think) that
there are any tafseers, etc to some holy books.

so basically (to me) its practically fruitless (other then the fact you can learn from other people how to debate and what points should be bought up in a debate) since its like going around in circles and i just tend to think "well whats the point, its not like they are basing their answers on anything truthful anyway "

secondly, i feel that even when i do ask questions, its just a whole preaching of their own religion (which i tend find extremely annoying).

does anyone else see appoint in these debates from an Islamic point of view, are they useful, encouraged, pointless, etc.

do people actaully learn somethign from this...has anyone felt that thier own heart have become leniant/understanding of Islam because of such debates
becuase to me when you are arguing/debating you're simply trying to put your point across and counter the other one which =/= people actually listening and understanding your point of view.

p.s i dont mean this to be a bash at Christianity, im just using it as an example since im more familiar with that religion than any other one.