Apparently, the previous version aimed to be able to handle the situation when you run mysql_secure_installation from inside the basedir. E.g. you have
basedir/bin/mysql_secure_installation
basedir/bin/mysql
basedir/bin/my_print_defaults
you do
cd basedir
and you run
./bin/mysql_secure_installation
Naturally, basedir/bin is not on the path, and you did not provide --basedir.
So, it would search in ./bin and ideally, would find stuff there; only this logic did not work anyway. Now it's gone, and if there is no basedir, the script immediately defaults to the bindir which was set by cmake.

On the other hand, I cannot think of a case when this intermediate logic would actually be useful for practical purposes.
If it's a source build, there will be no ./bin dir, all stuff is in ./scripts or ./client or ./sql, so it would not work..
If it's an installed build, then the cmake's bindir is just as good.

Source build/installation – looks ok after a quick sanity check.

Centos7, Debian Jessie installation – looks ok after a quick check for regressions

Quick check did not reveal any issues. It's possible that there are configurations that I did not consider, apparently we will have to rely on community testing for those.