According to, IIRC, either Randy Russell or Alan Cox a decade ago, /opt was added to the FHS at the request of the proprietary Unix vendors. The idea of 'optional' 'third party' software is ridiculous in an Open Source OS. Throwing binaries, libraries, config etc. outside standard folders means long library paths, long binary paths, config that can't be backed up from a central location, etc.
Eg, say Debian include Chrome (as Chrome, not Chromium) in their next release. It will move from where it is now to the normal locations for unimportant software beneath /usr.
That said, I don't really care that much anymore. There's very little expectation for quality finished software on Linux in general:
* Most apps aren't packaged, even for the two major distros people use
* half of GUI apps still don't install with shortcuts
* WMs still invent their own .desktop replacements
* package descriptions for 'foobar' are 'the foobar package' or 'libtoolkit GUI for libfoobar'
* service descriptions are 'starts the foobar service'
* man pages either don't exist, point elsewhere or consist of 'debian says we have to have a man page, this is a man page, alas it has no content'.