Servant is actually a literal translation of the greek diakonos, origin of the English word 'deacon.' Overseer is a very legitimate--probably the most accurate, in fact--translation of the greek episkopos, origin of the English 'bishop'.
The word 'bishop' implies a clergyman who has oversight over several congregations, so I think that the WTS is fully justified in not using it to refer to their elders. The term 'deacon', however, describes exactly the same thing as 'ministerial servant.' I think that the only reason the WTS doesn't use it is because they want to seem different from other Christian religions. (The term 'deacon' was used back in Russell's day, btw. It was eliminated at the same time that Rutherford got rid of the elders.)