Use CommandExecutor by default on plugins :P

src/pocketmine/command/PluginCommand.php

@@ -39,11 +39,7 @@ class PluginCommand extends Command implements PluginIdentifiableCommand{
 	public function __construct($name, Plugin $owner){
 		parent::__construct($name);
 		$this->owningPlugin = $owner;
-		if(!($owner instanceof CommandExecutor)){
-			trigger_error("Plugin does not implement CommandExecutor", E_USER_WARNING);
-		}else{
 		$this->executor = $owner;
-		}
 		$this->usageMessage = "";
 	}
 

src/pocketmine/plugin/Plugin.php

@@ -23,13 +23,14 @@
  * Plugin related classes
  */
 namespace pocketmine\plugin;
+use pocketmine\command\CommandExecutor;
 
 
 /**
  * It is recommended to use PluginBase for the actual plugin
  *
  */
-interface Plugin{
+interface Plugin extends CommandExecutor{
 
 	/**
 	 * Called when the plugin is loaded, before calling onEnable()

src/pocketmine/plugin/PluginBase.php

@@ -22,9 +22,7 @@
 namespace pocketmine\plugin;
 
 use pocketmine\command\Command;
-use pocketmine\command\CommandExecutor;
 use pocketmine\command\CommandSender;
-use pocketmine\command\PluginCommand;
 use pocketmine\command\PluginIdentifiableCommand;
 use pocketmine\Server;
 use pocketmine\utils\Config;
@@ -152,6 +150,18 @@ abstract class PluginBase implements Plugin{
 		}
 	}
 
+	/**
+	 * @param CommandSender $sender
+	 * @param Command       $command
+	 * @param string        $label
+	 * @param array         $args
+	 *
+	 * @return bool
+	 */
+	public function onCommand(CommandSender $sender, Command $command, $label, array $args){
+		return false;
+	}
+
 	/**
 	 * @return bool
 	 */