In my opinion we have two very different types of 'contact me off list' things going on here.
We have commercial solicitations and people looking to make contacts for buying transport circuits, capacity, etc.
And then on the other hand we have 'contact me off list' asks related to network operational issues, when the subject pertains to what's going on inside a particular AS, or in some peering or traffic routing problem. Not everybody wants their own or their peer's dirty laundry aired in public if something is broken or misconfigured in their relationship to the rest of the global routing table. And particularly not in a context where it will go in a public archive forever.