I accept your point Delly, but sentencing is a separate issue and nothing to do with a Jury. They are people with probably similar experiences as your own and could be influenced by knowing of a past conviction when they are reaching a verdict.
Say you had been fined for speeding in the past and another fine would take you into the points situation where you could loose your licence. On this occasion a mistake has been made and you weren't speeding at all, would you think it fair that you lost your licence because of past convictions?