I don't think it's good idea, considering it could be surprising. What user installs is more important, than what operating system installs in my opinion. Consider you use tool like ccache – for it to work, you have to put it in $PATH before actual gcc. With current behavior, the standard gcc will run, as it's inside /usr/bin.

Or, I'm going to give other problem. Suppose you compile something you have already installed on your computer, for example Perl, or something (or even, update the standard modules that contain binaries with versions from CPAN, without root access), but with newer version. You add it to path, but it doesn't work, as /usr/bin contains Perl.