In wotr there can be more players on maps than in skirmish. Therefore, it might happen that there are 3 players, but only 2 castles. Then one player can't do anything at all.
Also, with castles players would always need siege engines, which should only be necessary on fortress maps.
Furthermore, building huge armies in real time battles completly contradicts the concept of WotR. Players should build there armies on the worldmap and only strengthen them a bit in the real time battles.
Therefore, it won't be changed.