This would avoid a HTTP request per YUI files and would greatly improve the bootstrap of the PlatformUI App.
References:

• The non merged PR on ezjscore that tries to the same https://github.com/ezsystems/ezjscore/pull/17
• combine and comboBase config option of the YUI Loader http://yuilibrary.com/yui/docs/api/classes/config.html#property_combine
• unmaintain combo.php in PHPLoader project from YUI: https://github.com/yui/phploader/blob/master/phploader/combo.php

Alternatively, for the PlatformUI, the files referenced in yui.yml and css.yml could be minified and merged together.