"Agreeing" is about action and "Agreeable" is about attitude. It's no accident that one of the key Bible verses that talks about defending the Christian faith involves both action and attitude. "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have, but do this with gentleness and respect." (1 Peter 3:15) We are not called to agree with those we disagree with (which makes no sense because it's nonsense). Yet we are called to be agreeable, since what you say is easily undermined when you say it the wrong way!
We ought to affirm those who say we should respect others - it's a good thing, but point out this does not entail that we agree with another person's outlook on life or the choices they make. In fact, we demonstrate greater respect for someone when we tell them we do not agree because we're trying to steer them in the right direction.
A news story reported that a woman became very sick after spending years drinking only diet soda. If this was someone close to you, you would not stand back because you are the kind of person who respects everyone's beliefs. You would share the dangers - and introduce the benefits of drinking water and natural juices. Your respect for this person would not deter you from openly disagreeing with her, it would encourage you to reach out to her.
One of our greatest challenges is standing up for what we believe (and openly expressing areas where we do not agree) while remaining agreeable. Yet every Christian is called to share the reason why a biblical outlook on life is better - it resonates with reality - in a spirit of love and compassion. That's not easy when we engage Atheists, Satanists and Wiccans, but Jesus is the ultimate example of what it looks like to connect and communicate with broken people without compromising your faith.