The point of a layman jury is to prevent the judge from going too scientific on shit and producing results that are not connected to the real world. Which is a real possibility. I know that legal experts sometimes get fascinated with the unreal quirks legal systems can produce. It leads to weird and unexpected results once in a bluemoon. Laymen are there to prevent this from happening. They're there to say "Hold on, this isn't some academical question, this is a real life case... you need to step back and take a look at what you're about to do".
And for that, their input is very valuable. The problem we have these days, however, is that their opinion seems to get way to much weight in cases that are complicated and the weird result is actually the expression of actual justice in a complicated way. While jurys need to refrain the judge from going weird places with his legal interpretations, there are cases when a judge needs to be left to his own devices and fit a legal system onto a case without precedence or any actual legislation suitable for it. And sometimes a weird result needs to happen just so the case can be taken up with the higher courts (ultimately the supreme court) just to get the highest possible authority to make a basic ruling everyone else can take as precedence. Laymen don't understand these intricansies of legal systems and why sometimes stupid rulings have to happen for the greater good.