I would not say a diversity in length is a true necessity when analyzing the par 3's of a golf course, but I will say that can factor into my opinion of courses. I suppose with very good terrain and other natural features, you could design all the par 3's to be the same length and they all be interesting and good, but its not likely. Variety is a very good thing for par 3 holes, and all holes for that matter. Ideally, I would like to see One par 3 in the range of 230 yards, One around 130 yards, and the other two around 175, but very different from each other. This, of course can't always happen, but its great to see variety.
--On the same note-is it not good to see diversity in the lengths of the par 4's and 5's as well?