A small, or seemingly small sin can ruin your testimony. People often say "What do you expect of me? I'm only human, I can't be perfect," when defending their sin. But if one truly loves God, if one truly desires to serve Him and see Him glorified how can we say things like this with a straight face? If one truly loves God won't they be concerned that their unrepentant and purposeful sin would mar their testimony?
Dead flies cause the ointment of the apothecary to send forth a stinking savor: so doth a little folly him that is in reputation for wisdom and honor. (Ecclesiastes 10:1, emphasis mine)
Small cancer cells bring death to the entire body if not removed or healed. A small crack in a damn left unrepaired leads to a great flood. This is why Scripture speaks so often of purging ourselves of old leaven. To purposefully allow old habits to stay opens you up to allowing more and more sin in, and what will that say to the unbelieving about the transforming power of Christ?
A little leaven leaveneth the whole lump. (Galatians 5:9)
Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us: (1 Corinthians 5:6-7)
And besides, did not Christ Himself tell us to be perfect as our Father in heaven is perfect (Matthew 5:48)? Obviously one is still going to have their momentary slip ups, but I believe this is a call to rely upon God and allow ourselves to be purged of our old natures. Similarly one mustn't put themselves in a position to be tempted. To avoid temptation, to avoid giving the enemy a foot hold in our lives, these are things we can and must do. To purposefully walk in a way that will make sin more easy to enter into is foolishness.
If so be that ye have heard him, and have been taught by him, as the truth is in Jesus: That ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts; And be renewed in the spirit of your mind; And that ye put on the new man, which after God is created in righteousness and true holiness...Neither give place to the devil. (Ephesians 4:21-24, 27)
For this is the will of God, even your sanctification, that ye should abstain from fornication: That every one of you should know how to possess his vessel in sanctification and honor; Not in the lust of concupiscence, even as the Gentiles which know not God: (1 Thessalonians 4:3-5)
For our conversation is in heaven; from whence also we look for the Savior, the Lord Jesus Christ: Who shall change our vile body, that it may be fashioned like unto his glorious body, according to the working whereby he is able even to subdue all things unto himself. (Philippians 3:20-21)
As followers of Christ we are called to live a life of separation, a life different than those around us. To put ourselves in situations where it is likely our walk could falter is to go against Scripture which commands us to have no confidence in our flesh (Philippians 3:3). To open ourselves up to temptation is foolish and unnecessary. No man, or woman for that matter is an island. Sin one does behind closed doors will inevitably work it's way into the light and will corrupt the believer through and through if left unchecked and unrepented of. If you the believer can't walk in a way that shows God's power, who will? A little sin hurts you, and hurts those around you in that it can damage other people's perceptions of who God is. And it is, after all, about bringing God glory.