SERM is required, as Phera reduces your testosterone levels, and unbalance them with estrogen levels. If you look at the picture in other way you'll easely see that you should always prefer use a SERM than an anti-estrogen. Imagine that you just low your testosterone to a point where it is on the low range, or even a little below the normal range, of the normal levels, and your estrogen is in the right place, what you want is just to bring up test and keep estrongen right where it is. If you use an anti-estrogen you will boost your test, but will mess with estrogen values, bue if you use a SERM you'll selectively boost your test while keep your estrogen as stable as possible.
So in simple terms: A.I. brings less stability to your hormone levels than a SERM, and is stability that you should be looking for after a cycle.