One exception I can think of though is when previous conviction changes the nature of whether or not a charge should involve premeditation. If you have been convicted of DUI previously, taking a drink without handing over your car keys is premeditation. So the charge might be different.

vis somebody with bipolar disorder who commits a violent crime without meds. Second time, the failure to stay on meds should change the charge, since that act was taken while competant to make a decision (the decision to go off med being a predicate act)

>>>Why should his previous convictions not be used in court? I would think that they are very important. Our justice system has some very obvious flaws.
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>>>http://www.wral.com/news/local/story/3218206/
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>>There isn't enough detail in that news report to tell at what stage was the evidence used. IANAL, but as Alan points out, it seems fair to me that prior convictions shouldn't be used during the trial; prosecutors should prove you did THIS crime, not that you might have because you've done it before. However, and I believe this is the case, prior convictions should be used during sentencing (ie. once you've been found guilty), since the punishment should go up when you continue to break the same laws.
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>Agreed! Take that into account and punish them more harshly. But prejudicial "evidence" should not be presented in the current case.