Because data modification in plugins is nicer than just an emit listener. Note: your listeners have to be specifically adapted for plugin.hook() forwarding

plugin.js

var EventEmitter = require('events').EventEmitter,
    util = require('util'),
	async = require('async')

function plugin() {
	EventEmitter.call(this)
}
util.inherits(plugin, EventEmitter)

// A horribly butchered form of EventEmitter.emit, hacked to support data modification & chaining
plugin.prototype.hook = function() {
	var handler, args, listeners, final, type, self = this
	args = Array.prototype.slice.call(arguments)
	final = args.pop(), type = args.shift()

	// check to see if we have a finale...otherwise, derp out.
	if(typeof final !== 'function')
		throw new Error('no final handler defined')

	if (!this._events)
		this._events = {}

	handler = this._events[type]
	if (typeof handler === 'undefined') {
		done.apply(done, args)
		return false
	} else if (typeof handler === 'function') {
		async.waterfall([ function(done) { handler.apply(self, args.concat(done)) } ], final)
	} else if (typeof handler === 'object') {
		listeners = handler.slice()
		listeners[0].bind.apply(listeners[0], args) // easy solution to prepend the initial args into the first listener call
		async.waterfall(listeners, final)
	}

	return true
}

module.exports = new plugin()