Why evangelize?

I am taking a course on the Biblical and Theological Foundation for Mission right now. For class I am reading Newbigin's book "The Open Secret." He has an interesting quote in one of the first chapters. I am sure I am no where near the intended point by not going further into his argument with you (he soon after begins to help you in responding to questions such as these - I am not satisfied with the answers). But I think these are questions worth asking - at least of myself:
Why not join with the sincere adherents of all religions in seeking the fullness of the truth to which they all aspire?

Why not join with all people of goodwill in tackling the real human problems of hunger, oppression, sickness, and alienation, instead of seeking more adherents for your religious group?
How would YOU respond to such questions?