There are several reasons to finally require slasharguments and get rid of the setting:
Files (images, uploads etc) are provided via a script using 'slash arguments'. This method allows files to be more easily cached in web browsers, proxy servers etc. Unfortunately, some PHP servers don't allow this method, so if you have trouble viewing uploaded files or images (eg user pictures), disable this setting.
First: it will ease some new feature development (such as JS framework). Besides, Moodle already has several areas that do not work correctly with slasharguments disabled.
Second: if Moodle prevents installation on the server that does not support slasharguments, it will force people to read documentation and set up the server properly instead of creating the forum posts AFTER installation asking why does not CSS load for them.
The question is - do you know of any [modern] server which it is not possible to setup to use slasharguments? If we require it, what can we potentially break? Can it prevent you or somebody you know from upgrading?
Should we extend existing documentation in Moodle to cover more cases when slasharguments might not be working by default?
At the moment docs are: