Hi, I think reed-write is an useful plugin that helps non-developers defining content types, and at work we used it for many sites.
Is this file really necessary? Is there a way to disable the loading of this file? If this file is necessary, shouldn't you embed the script content in the head, or generate this only when it changes and cache it as a static file, instead of use this way that makes WordPress load twice at every page viewed?
On a site I tried to use the plugin W3 Total Cache, but apparently _rw_script.js.php is not cached, so even if I load quickly a static cached copy of a page, _rw_script.js.php is reloaded anyway in any page, slowing down the load time and nullifying the work of W3 Total Cache.
Apparently this scripts allows to make AJAX requests to _rw_query.php, to make "wp_queries". I think this functionality is not used by many reed-write users, and some may even be unaware. Moreover, AJAX requests should be handled through wp-admin/admin-ajax.php and hooks 'wp_ajax_ACTION_NAME' and 'wp_ajax_nopriv_ACTION_NAME' as explained on http://codex.wordpress.org/AJAX_in_Plugins
Shouldn't you make all this optional, or even split this feature into another plugin?