The point you all seem to be missing is that if a person has been convicted in the past for a crime of whatever description, they have been punished for it already in a court in a court of law. You can't punish someone twice for the same offence which is what in reality will be happening if the prosecution is allowed to use this information. His past offences are of no relevance to the case that is being heard until it comes to sentence. The jury should make their decision on the facts of that particular case placed before them, not be influenced by what has happened in the past.
As to the lawyers for the defence, of course they try to put the opposite side of the picture, that is what they are there for after all. Wouldn't you expect your lawyer to plead your case if it were you in the dock?

You have a poor view of people if you think a jury can be 'duped' by the defence lawyer, but if that is so can't they also be 'duped' by the prosecution or is that right in your eyes?