I have Frank’s timing fixture for the 5-18 hp OMCs and I have found it to be a worthwhile investment if you regularly work on these motors. With the right setup it is faster and more precise to use than a feeler gage. I have done enough tune ups on these and other engines that setting points with a feeler gage is fairly quick and I can get good results, but even then I find one off a bit if I go back and check it with the fixture.
Is the fixture necessary to get a good running motor, no. To me is is a time-saver and you know you have set the points for optimum performance. It removes the guesswork from setting the timing exactly 180 degrees apart. That gives the best possible idle quality.
-Ben