Trash purge is typically a function that could be automated.

Most applications propose such a functionality (i.e. my webmail let me configure it to automatically delete messages in the trash after 7 days)

At present time lots of people get bored with emptying the trash so they prefer definitely delete files (shift+suppress)
But sometimes they make a mistake and regret it : they've lost the way to restore the file by refusing using the trash
See the paradox : the trash is a functionality that people want but refuse to use. It could be the sign that the functionality has usability drawbacks and that it should be improve...

Moreover people doesn't want to empty the trash in real life : why would they enjoy doing it on their computer ?!

The improvement could consist in automatically suppressing the files in the trash can after a couple of days. Of course it should be optional

Some people are using cron a rm -rf ~/.Trash/* but that is not optimal
See also that Python script http://www.logfish.net/pr/autotrash/

See also
http://open-source.ecchi.ca/?voir=articles/dont_care_about_trash
http://en.opensuse.org/OpenSUSE_GNOME_post-10.3_ideas#Misc

Thanks