Effective defence would allow the guilty to go free. (although “guilty” is a jury verdict so that is impossible but I presume you mean would be found guilty had more evidence prevailed). You can’t have an advocate that draws arbitrary lines at which they are not on your side advocating for you, what would those be?

Say someone says to their lawyer they shot the person but really they are covering for their wife even to mthe lawyer. The lawyer then hears them say in court they didn’t shoot (the truth) but the lawyer just knows they are saying something different now to what they said before. It could be that.