We already have a system to make sure laws remain appropriate: it is called an elected legislature.
They system you are suggesting would in effect turn every trial into a little election on what the law should be, but with only a small fraction of the electorate voting, and the results only applying to that one case.
Checks and balances considered useful. I always want the option to exist, even though people won't need it in most cases.
And also, every case adds to the case history; it doesn't take many cases to create a pattern. So no, the results don't only apply to that one case. And even if they do, a single instance of well-applied nullification might not mean much to the general public, but it means the world to the defendant.
They system you are suggesting would in effect turn every trial into a little election on what the law should be, but with only a small fraction of the electorate voting, and the results only applying to that one case.