Is the choice of test statistics in hypothesis testing a completely philosophical one? Is the choice of test statistic in hypothesis testing a completely philosophical one? In other words, is the choice of test statistic and rejection/acceptance region a completely judgement call and is not bounded by any requirment?
 A: I am assuming that you are asking about the choice of test statistic within a specific statistical model rather than asking about the choice of statistical model. I am also assuming that you are asking about the test statistic to be used in a classical hypothesis test in the accept/reject manner.
The choice of test statistic is made on the basis of the properties of the resulting test. There is good reason to choose the test statistic to optimise the power to discriminate between a true and false test hypothesis, but it is also useful that the distribution of the test statistic be known. 
Student (Gossett) wanted to devise a significance test for means from small samples. His resulting t-test uses a particular test statistic, Student's t, not because he wanted to test the ratio of the mean and standard error, but because the distribution of that test statistic is derivable. 
Whether you wish to call the choice of test statistic a "philosophical one" depends on what you mean by that.  ;-)
