If a defendant is not guilty, how would not having tougher standard of proof affect this person?
If I understand your question correctly, you are asking how an easier standard of proof affects innocent defendants. If so, having an easier standard of proof would tend to make it easier for an innocent defendant (or a guilty one, for that matter) to be convicted of a crime.
From a defendant's point of view, a tougher standard of guilt is a good thing. If you are accused of a crime, you would rather have the government be forced to prove you guilty beyond a reasonable doubt, or beyond any doubt at all. You would not like the standard to be a simple preponderance of the evidence.
So an easier standard of proof would be very bad for an innocent defendant.