Many lower-key evangelism styles operate under the same methods. While never explicitly stated, the logical conclusion of this style, even sans-picket-signs, is, "If you have doubt or still sin, then you are obviously not a good enough Christian to earn God's grace."
Which is absolutely ludicrous.
First, we do not "earn God's grace". It is grace because it impossible to earn. Our sins were paid for by the death of the Son of God. You can never earn something bought at so high a cost. Yes, the acceptance of grace costs us our lives, as Dietrich Bonhoeffer reminds us. But even this surrender does not "earn" grace, but instead helps us to fully embrace it. Likewise, Bonhoeffer also reminds us of Luther's famous saying, "Sin, and sin boldly; but more boldly still believe in Christ and rejoice in him." Bonhoeffer cautions us, "[D]on't try to become what you are not." For the very second we try to claim that we can achieve perfection on our own accord, we instead claim a state of arrogance and, in this state of arrogance, reject the very grace that makes us perfect.
For example, if I claim that, after becoming a Christian, I ceased to ever give in to angry thoughts, I would be a liar and a braggart. Granted, acceptance of and discipleship to the Risen Christ will lead to a more peaceful state of mind. Over time, as I begin to walk with the Lord and draw more closely to Him, my mindset is changed. But, to be blatantly (and boldly) honest, I still get angry over small things. At my worst, I am an angry person - at my best, I am a sinner redeemed through the blood shed on the Cross.
And I would go so far as to state that only through openness about our shortcomings that Christians will ever shed the image of hypocrisy that so plagues us today.
Secondly, doubt does not negate faith. Only through doubt is faith truly faith. For the very instant you remove all doubt, faith - the conviction of things not seen (Heb 11:1) – is no longer faith and enters the realm of the empirical. People do not have faith in the existence of coffee cups*; cups are proven daily to exist through direct observation and interaction. God, though, exists beyond our senses and our understanding, known only through faith. To claim a complete understanding and knowledge of God either A) claims the impossible - a finite being understanding an infinite deity, or B) limits God. Christians should do neither. Indeed, doubt strengthens faith. Instead of dismissing doubts as unfounded or false, true faith confronts doubts; it struggles with them and forces the faithful to depend more heavily on God.
In order to encourage and cultivate faith, we must also radically change the field of apologetics, as well as recognize that the sole use of apologetic arguments do not constitute evangelism. As I was once told at an FCA camp, "You cannot argue someone to Christ. If someone is convinced by scientific arguments alone, they can just as easily be convinced by counter-arguments." We can no longer devote ourselves to "proving God". Instead, we should focus on finding Him, walking with Him, and loving Him.
Love - now there's a concept. If instead of making everyone feel guilty about not being perfect, perhaps we could show them God's love. If God is love, then to show love is to show God. If we were to focus on relationships and servanthood, if we were to relate our own stories of God's love...well, wouldn't that be something?
*Even David Hume, the father of modern skepticism, was willing to accept empirical evidence - though he did acknowledge a faith in empiricism. This, though, is better suited for discourse concerning the "New Atheism".
Post Script: I owe a tremendous amount of thanks to my friend Suzanne. Without her editing...well, something about weak verbs, "this" not modifying anything, and comma splices.