Many years ago there were times that people wanted Firefox on Debian, so do I. This was (maybe) good reason to add Ubuntu repo to SourcesList but I don’t think that mixing Debian/PureOS and Ubuntu repos (or vice versa) is adequate approach nowadays (as big number of the .deb packages have different names, although they might look like they are the same, but they are not, etc.). Thinking broad is fine, but relying on one distro might make things consequent and consistent.