works on my machine
They technically do, but let's say I want to have kubectl v1.10 and go v1.0 among others. Some packages do get packaged multiple times for each version. Most don't. Then you end up hunting down build hashes for each version using tools like https://lazamar.co.uk/nix-versions/ which work I guess but the UX is not great. Talking about this with my colleagues, most won't bother until you can declaratively list tools and versions directly
works on my machine
probably until it is easier to pinpoint specific versions of any package in question