BuddyPress.org

Well, the skeleton uses WP_PLUGIN_URL because custom components should not run in the /buddypress/ directory. Since BP_PLUGIN_URL is defined in bp-core.php (line 9):

define( 'BP_PLUGIN_URL', WP_PLUGIN_URL . '/buddypress' );

if you use BP_PLUGIN_URL, it will be pathed incorrectly.

So, the question is this: Where do you have your custom component installed? It should be in /plugins/ and not /plugins/buddypress/