In Season 3 Morocco was added as the fourth filming location used to in Daenerys story line to represent the cities of Astapor and Yunkai.
As A Game of Thrones keeps growing and expanding, adding new characters and an ever thickening plot, the cost of production grows along with it. So the rumor that the production team is not returning to Morocco for Dany's scenes comes as no surprise. But don't worry the Morocco will be replaced by locations in Croatia, most likely around the city of Split, for Dany's scenes.
I think this is a good choice in the grand scheme of things. Mainly because it will cut back filming costs, which will allow them to spread the money around to other parts such as casting new characters, CGI, and of course a large battle scene at the end of the season. It will also reduce the hassle of sending part of the Game of Thrones crew half way around the world.