I was wondering the same. I wrote "What good writers we are."... not good :P
It's funny because it could be understood your way, but that would be an archaic, today not used form of the Hungarian sentence (now it is "Milyen jó írók vagyunk"). Actually here the "mi" simply means we, and it is not necessary.
When learning Hungarian they often add the pronoun even when its not needed. Maybe to help us in our learning and to be understood. It can be added in for emphasis but it isn't for this sentence.