I've previously mentioned in passing that I simply can't use Python 3 on some platforms because it's not there (or perhaps it's there only in an old and lacking version). As reported by Twirrim, in some places the popular answer to this issue is to say that I should just compile my own local version of Python 3 by hand (perhaps in a virtualenv). At this point most of the sysadmins in the audience are starting to get out of their chairs, but hold on for a moment; I want to make a general argument.

There is a spectrum of Python coding that ranges from big core systems that are very important down to casual utilities. For something that is already big and complex, the extra overhead of compiling a specific version of Python is small (you've probably already got complex installation and management procedures even if you've automated them) and can potentially increase the reliability of the result. Nor is the extra disk space for another copy of the Python interpreter likely to be a problem; even if the disk space used by your system doesn't dwarf it, your core system is important enough that the disk space doesn't matter. But all of this turns on its head for sufficiently little and casual utilities; because they're so small, building and maintaining a custom Python interpreter drastically increases the amount of effort required for them as a total system.

Somewhere on the line between big core systems and little casual utilities is an inflection point where the pragmatic costs of a custom Python interpreter exceed either or both of the benefits from the utilities and the benefits from using Python 3 instead of whatever version of Python 2 you already have. Once you hit it, 'install Python 3 to use this' ceases being a viable approach. Where exactly this inflection point is varies based on local conditions (including how you feel about various issues involved), but I argue that it always exists. So there are always going to be situations where you can't use your own version of Python 3 because the costs exceed the benefits.

(With that settled, the sysadmins can now come out of their chairs to argue about exactly where the inflection point is and point out any number of issues that push it much further up the scale of systems than one might expect.)