Quite often stores want to restrict checkout to only in stock products (Settings -> Options -> Stock Checkout = No), but at the same time they might have products coming soon, or products only taken in on order (?Delivery item?) - which they also want to accept orders on.

Thus I believe that the stock statuses should have an option to set and control (override) "Stock Checkout", or maybe better, add this as an option to the products data (Always allow checkout) per product.