> under what circumstances today is it the best thing to spend routable IP addresses for machines that aren't exposed directly to the Internet
Under the circumstance that you have a spare /8 you aren't using for anything.
You could argue that "the best thing" in that case would be giving, selling, or leasing pieces of the /8 to someone who needs the space. But maybe you aren't in the philanthropy business, so giving's out.
And maybe the prices aren't high enough for your taste, so selling or leasing are out. Or maybe there's simply not enough demand on the market to absorb 16m addresses. Or maybe the terms of the allocation agreement say you're not allowed to transfer them, and you're afraid that trying to do so will give the Internet people a justification to give them to someone else.
> if routable IP addresses were easy to get, there would be lots of cases where it would make sense to use them
If your organization has a spare /8, routable IP addresses are easy to get for you, so these cases do make sense, for you.
Under the circumstance that you have a spare /8 you aren't using for anything.
You could argue that "the best thing" in that case would be giving, selling, or leasing pieces of the /8 to someone who needs the space. But maybe you aren't in the philanthropy business, so giving's out.
And maybe the prices aren't high enough for your taste, so selling or leasing are out. Or maybe there's simply not enough demand on the market to absorb 16m addresses. Or maybe the terms of the allocation agreement say you're not allowed to transfer them, and you're afraid that trying to do so will give the Internet people a justification to give them to someone else.
> if routable IP addresses were easy to get, there would be lots of cases where it would make sense to use them
If your organization has a spare /8, routable IP addresses are easy to get for you, so these cases do make sense, for you.