Keep in mind that these schemes are mainly meant for parsers. That's also why standards are important.
youre very right , especially on tel:+43-7654321 - real-world-examples: skype-link, wtai:// so probably a parser (and if only for supporting simple-minded like me) would do better if the common known "://"-string is also accepted - no big deal so far.
now I knwo, Luke implemented a URI handler (acc to ~half year ago) - question: why isnt it in use (by shops) already any examples that do?