It depends on your task. For example, if you want to poll all lifetime of your client application, I would go with higher poll interval (several seconds) or go with socket connection. But if your task to send something and wait for completion of async process on server-side, polling will be a nice approach. And in your PollingService, wait for response from the server, and after 500ms timeout, do next request, you will not have any synchonization problems at all.
//delay 500 ms between response from the previous request and future request
var service: PollingService = new PollingService(500);
service.start();
For realisation of such service you will need only 2 classes: URLLoader and Timer. And maybe signals or EventDispatcher ;)