I agree that someone should not PUSH their views on others. However I do not believe that sharing your faith is a waste of time. We can learn much by sharing together. It is when others believe they know it all, and want to try to force you into their faith, then they are doing wrong.
Sometimes it is necessary to speak to someone about what they are doing, if the Lord leads you to say something. BUT ... you should also wait on the Lord to tell you WHEN to say something. If you say something in your own timing you could actually push them AWAY from God, and not lead them to Him.
Everyone has their own beliefs, and no one should down them for them. If they feel the person is wrong, they should take it to the Lord in prayer, not beat them over the head with belief systems.
Often Christians become over zealous to witness to others, this is because they feel they have found something awesome, and they want to share it with the whole world. BUT ... they need to become consistent and deeply rooted in their own faith, before even trying to witness to others. Some go all out after being saved [born-again] and try to change the world for God, but what they don't realize is that we all have Free-Will given to us by God, to make our own decisions and draw our own conclusions. If we then have Free-Will they must realize, they can't change ANYONE, it is God who changes people. And if He changes people, He still allows them to want that change. He does not change anyone who does not want to be changed.
~Jewel