This is why we can't pre-emptive strike NK - unless we're willing to write off Seoul, South Korea, and everyone who lives there along with the SEA economy.
Would we really sit around and do absolutely nothing to stop North Korea from destroying Seoul? I imagine that it would take a considerable length of time to actually accomplish such a feat, even with 250,000 tubes and launchers.
All it takes is for one or more of those shells to have a WMD warhead.
We can't stop them all, and it's certain that they have *at least* one WMD (bio or chem) warhead loaded...