I think it might depend on what sort of problem you're trying to solve.
If you have, let's say, a django app, and you want to be able to run it all sorts of places, Docker is very much the wrong tool; it doesn't run at all most places, and it's finicky to get working. You're better off just getting that one app to run when and where you want. And if you run into any issues, virtualenv will solve it, no big deal.
If you have a bunch of apps you want to get running (or perhaps a bunch of interlocking pieces of a single stack, or the different elements of a SOA), then Docker suddenly starts to look very attractive. And then you might go to the trouble to get a single gold server image with docker installed and working (or an Ansible playbook, or a Chef cookbook, or a Digitalocean snapshot, or an EC2 AMI, or whatever), and you know you can just spin up a server and deploy any app you want to it. And once you start thinking about testing, CI, orchestration, automatic scaling, etc., it all becomes that much more attractive; you've got these generic docker servers, on the one hand, and these generic docker containers on the other, and you can mix and match them however you like. When you start having more than 1 server and 1 app, that's amazing. Very much worth the cost of entry of having to install docker everywhere...if you need that kind of thing.
You're focusing on portability between operating systems, but that's not the point of docker; as you say docker isn't portable at all (which should be a strong hint that isn't the problem it solves). But docker containers are portable between servers with docker on it, and with some architectures (or at a certain scale), you will suddenly realise just how useful that is.
If it helps, consider Heroku (and the other PaaS outfits like dotCloud, etc.). A lot of startups outsource big chunks of their infrastructure to Heroku, and Heroku uses a very docker-like architecture. If you were to shift that back in house, in many cases that same architecture makes sense (largely depending on just what you were outsourcing to Heroku...). ...and sometimes it doesn't. But if it does, docker is probably a core part of any attempt at implementing your own in-house PaaS. And if you need that kind of thing, you aren't going to stop because "well, it doesn't run on OSX"; nobody (well, nearly) is using OSX is production. :)