$13K for a lens is just gouging.
I use the 400mm all the time, and now I just rent one whenever I need it. Bill it to the customer. Yeah, it would be nice to have one at home, so at a whim I could just go grab it and shoot. With renting, you have to plan ahead. It's never been an issue for me so far.
I need a 400mm not more than 10 times a year. I can see a 600mm even less use. If I was a sports guy or a wildlife, then I might consider buying it. It's just not in my budget at this point.
If I ever need a 600mm.. I would rent as well. Let's calculate. It costs about $300-400 to rent for about 2 days. That's means you can rent it for about 35 times before you would pay for it.
I have no problem if my employer buys one for us to use... but can see then renting.
$13K buys a ton of other gear I use more often...