As we have both seen The Mothership of Protoss is an ultimate War Machine and it can eridicate anything.But here some disadvantages the first one is its speed which is slow enough to keep the balance of races, secondly the cost will be very high and its Technological level need will be high too and third you can built only one which makes it a very important target and the losing chance of this ship will be increased.But I think adding this Machine in Starcraft 2 is wise.I belive this ship makes the game more exciting and fantastic.