Sometimes form can be relevant, to an extent. I suppose, after all, that's the motivation behind the "good character" defence. I was told of one case where the defendant was found not guilty altogether, because the jury was required to be unanimous but one old lady insisted that the way the boy looked he "clearly couldn't hurt a fly" -- despite all the evidence against him. Apparently, after the verdict was returned, the multiple previous offences were read out and this particular juror couldn't escape fast enough!
But that said, I don't think that making the jury aware of previous offences before reaching their verdict should be introduced as a matter of course. If someone has a string of previous convictions it might make it more reasonable to suspect them of committing this offence, but in the end the only thing that matters in reaching a verdict has to be the evidence for that particular case -- in particular if the offences are different anyway. The whole point of the jury system relies on the jurors being dispassionate, and motivated only by returning the correct verdict based on evidence. It seems to me that this principle doesn't hold if you enter knowing that he is "clearly a nasty piece of work" and therefore decide to find him guilty of... well, whatever he was hauled into court for, I suppose, and who cares what that actually was if you are convicting a man based on his past?
I suppose sometimes people who did what they were accused off get away with it on occasion as a result -- for sure, the system isn't perfect. But in that case you need better, or more convincing, evidence.