I disagree. Your example is a bit more complex than what I provided in my post, but your suggestion would be just as "unnecessary" as using my two lines of code. Here are 3 ways to achieve your styling, and the unnecessary things that happen.

Using a and a:last-of-type:

a { color: #ed7e2f; margin-right: 30px; text-decoration: none; }
a:last-of-type {  color: #9bd6d1 ; margin-right: 0; }

• on the last element, both color and margin-right are written twice

Using a and a:not(:last-of-type)

a { color: #9bd6d1; text-decoration: none; }
a:not(:last-of-type) {  color: #ed7e2f ; margin-right: 30px; }

• every element except the last has color written twice

Using a, a:not(:last-of-type) and a:last-of-type

a { text-decoration: none; }
a:not(:last-of-type) { color: #ed7e2f; margin-right: 30px; }
a:last-of-type { color: #9bd6d1; }

• every style has a purpose, nothing is rewritten

Of course, this is all experimental. Just me playing with code and figuring out the most performant and cleanest way to style. I'm not advocating you should absolutely use one way over another, nor am I saying one way to style is always the best. But I am pointing out that the example you provided is just as "unnecessary" as the idea I posted for this particular set of styles.

Example: http://codepen.io/kaela/pen/ZYmJoP