Hi Dear Veena.K, I think you could prolly make a case that in certain circumstances eavesdropping would be beneficial, and justified...maybe if you were really trying to help someone, or legally in criminal investigations or national security.
However, I myself just do not do it. I keep my life as simple as possible, and if I start to overhear a conversation, I just move away or if that is not possible, I let the people know that I am inadvertently hearing them.
Ever? Well, I suppose there are some circumstances when it's justified, but it's not something I would willingly do. Sometimes, if somebody is having a loud argument, it's impossible not to listen.