I think the original point of secret achievements were to protect the game's plot, such as "Killed <Insert Name Here>" or "Rescued the Princess". I don't have a problem with these kinds of secret achievements since you get them anyway as you progress through the game regularly. But I do hate the ones that require you to do something special or extrordinary, because the only way you know what to do is by looking them up on the internet, and even when you do unlock them, you can't really show them off because they always come up secret to your friends until they unlock it as well.
And to keep this post on topic, the secret acheivement in FlatOut is of the later variety....
Fight My Brute!