This started because Istredd's an overprotective fool.

Now he can't just go with his secondary plan of hoping Geralt could keep Dean on a leash and ugh actually trying to find a way to dispose of the Mark that Lucifer himself wanted nothing to do with after a point and why should Dean get an out a second time?

I have a middle man. As much as he could rely on Geralt for that much, but it's better than Castiel, and he doesn't have any other options. Trust me, I have no desire for tête-à-tête with Dean, and I'm sure he'd agree.