Presumably most civil servants are required to deal with the public at some point in their careers. Being able to speak the native language of the people you are serving would be advantageous. The more of the common languages in Taiwan that you can speak, the more likely this is going to be.
That is certainly a difficulty. Well, they already went ahead and had a Hokkien written section in the last exam. Being able to choose from Hokkien, Hakka, Aboriginal (and I think English is a good idea too) would be much fairer and try and foster a wider spread of language talents in the civil service. Perhaps having written sections in other Taiwanese languages would help act as an impetus for their standardisation.
Brian