Many Christians don't really even pay attention to the bible or even practice the religion. Heck, before my "falling from grace" all I thought was that there was a God that loved me and a Heaven to go to when I died (pretty much ignored the bible all together under the assumption that it was crap written by guys putting in their own agenda and calling it "the word of God"). Now that I don't believe in God, when I'm depressed and hopeless I wish I believed that at least I had someone who always loved me.
As for Athiests and agnostics generally holding Christan values, the same christians would probably still have the values they held as Athiests as well. You're right, it is reason that makes people respect one-another. Christians are just as likely to be jerks as a person in any other religion, people won't follow the commandments if it isn't convenient to them. Rarely have I ever seen a person say, "It's a sin to lie, so I'm not going to".
But as long as people actually ARE being perfectly decent to each other, does it really matter what they think is going to happen when they die? Just as long as they can be respectful, the motive isn't important. As long as the person is content in their belief in God, why hurt them by saying, "Reason is greater than faith!" when they would be the same kind person even if they were an athiest?