The difference between engineering and a trial however, is that when determining the result of a trial, the Prosecution has to use evidence directly related to the current crime they are prosecuting. They can't say "But this person has done all of these things, so he must be guilty," which is what introducing previous criminal history is doing. It's a form of poisoning the well. "See he's a bad man, he must have done this too."
This is why things like previous history are suppressed, because things like poisoning the well, and ad hominem attacks are, unfortunately, often persuasive to those uneducated in debate and logical fallacies, otherwise people wouldn't resort to them so often. Allowing that sort of poor logic into the court system will result in failures to gain justice. After all, for example, just because the defendant has a record of holding up KFC's, doesn't actually mean that it was actually him that held up the KFC he's currently been charged with robbing. It's up to the prosecution to prove that independent of his past.
The fact that some people who wanted suppression removed so that certain verdicts would have been different, can't understand that this sort of information is suppressed precisely because it has such an ability to change verdicts when other evidence of a crime isn't there, all while claiming that the jury would be clever enough to determine what was relevant and what wasn't, just reeks of irony or perhaps idiocy.