Data from the query string and the post body are aggregated into the request parameter set. Query string data is presented before post body data. For example, if a request is made with a query string of a=hello and a post body of a=goodbye&a=world, the resulting parameter set would be ordered a=(hello, goodbye, world).
So what is the issue here? It is quite simple really, if you POST to a Servlet it should ONLY return the value(s) for the parameter that were part of the POST Request, ignoring the GET values! The same is true for the opposite.
Even PHP has gotten this right by seperating parameters out into the $_POST and $_GET globals (the use of globals here is a whole seperate issue)
So why is this a big issue? Well for one, it makes it much easier for would-be hackers to try to do mean things to your application. There are lots of reasons that this is a bad idea, but the main one is that when you are posting parameters to a servlet, a great deal of the time, you are posting operational information, which can be changed by adding a GET parameter to the URL, maybe. And that's the kicker, you really have no idea whether the parameter(s) you are looking at were passed in on the URL or were part of the POST without additional work.
I suppose there are ways around this that could be implemented into a wrapped request, but he fact of the matter is that this is something that absolutely should be part of the spec. It is no secret that a lot of people want this to be added, and frankly it really irritates me that the community has not listened to the user base in the respect.