It's not that different than refraining from using non-POSIX syntax in shell scripts that are meant to be independent of a specific flavour of unix, or sticking

It's not that different than refraining from using non-POSIX syntax in shell scripts that are meant to be independent of a specific flavour of unix, or sticking with standard C rather than making assumptions that are only valid in one compiler. There are shades of grey, of course. Bash is probably ubiquitous enough that it may not be a big issue if a script that's meant to be universal depends on it, as long as the script explicitly specifies bash in the shebang. Sometimes some particular functionality is not technically part of a standard but is widely enough supported in practice. Sometimes the standards (either formal or de facto) are expanded to include new functionality, and that's of course totally fine, but it's not likely to be a very quick process because there are almost certainly going to be differing opinions on what should be part of the core and what shouldn't. Either way, sometimes you want to write for the lowest common denominator, and moreutils certainly aren't common enough that they could be considered part of that.