Possible reasons for wanting generator expressions to be generators / coroutines rather than simply iterable iterators. In response to questions from Dominic Denicola in http://domenic.me/2013/09/06/es6-iterators-generators-and-iterables/

generatorsAsCoroutines.js

/**
 * Treat generators and coroutines the same
 * This means we need to treat all generators
 * as coroutines, rather than the other way around.
*/
function *loggingDecorator(gen) {
  let processed = gen.next(null);
	while (!processed.done) {
		console.log("INFO:", processed);
		let unprocessed = yield processed.value;
		processed = gen.next(unprocessed);
	}
}

/**
 * A simple echoing coroutine
*/
function *EchoServer() {
    let response = yield null;
    try {
    	while (true) {
            response = yield response;
    	}
    }
    catch (e) {
        console.log('Shutting down')
    }
}

// Now let's create a generator and a coroutine

let genexp = (for (x of [1, 2, 3, 4, 5]) x * 2);
let echoServer = EchoServer();
// Prime the coroutine
echoServer.next();  // {value: null, done: false}

// With the current implementation of generator expressions
// loggingDecorator doesn't have to care what it is passed
let wrappedGen = loggingDecorator(genexp);
for (let item of wrappedGen) {
    console.log('From for loop:', item);
    // INFO: {value: 2, done: false}
    // From for loop: 2
    // INFO: {value: 4, done: false}
    // From for loop: 4
    // etc.
}

let wrappedCo = loggingDecorator(echoServer);
for (let item of [1, 2, 3, 4, 5]) {
    item = wrappedCo.next(item);
    console.log('From for loop:', item);
    // INFO: {value: 1, done: false}
    // From for loop: 1
    // etc.
}

// Now give everything a chance to shut down
// We don't need to worry about what we were provided
// Generators will simply terminate, while coroutines can
// perform any cleanup they need to perform.
wrappedGen.throw(new SpecialCleanupException);
wrappedCo.throw(new SpecialCleanupException);