A person's past behavior is often a very good indicator of how he will behave now and in the future.
Unless we are concluding that people can't change (including you and I), then I can't comfortably judge a person on past actions. Yes, the past may be an indication of patterns, but I feel it is important to be familiar with a person's desire to change those patterns. I think we all deserve that opportunity.
Generally, I disregard the past and try only to judge a person's intentions by current actions and words (I kinda have a word usage fetish). How would any one of us like it if the world actually judged our current lives based on our pimply faced, awkward, and unpopular days in middle school? And, I wonder, how many people out there REALLY never made a bad choice or entered into a negative pattern of behaviour for a time? Why should one have to prove their redemption to anyone but themselves and God? Why should someone who 'fooled around' with 3 people just because they drank judge the one who knowingly 'fucked' 25 . . . sober? They both acted promiscuously.
I don't want to be preachy, either, but I feel that judging simply from the past is being blind to the person in front of you. As long as everyone is safe and healthy, going on about the past seems shallow and shortsighted, too me.
That's not to say, that one can't use past information to recognize re-emerging patterns.