Actually that isn't true.
AI's are superior to SERM's, both during cycles and as PCT.
People are just too conservative to accept the fact that newer and better anti estrogen compounds are available today. Sure, Nolvadex and even Clomid were good in the old days, when AI's weren't available. But times have changed now.
Using receptor blockers is like comparing a guy walking into a warzone wearing body armour, to a guy avoiding the warzone entirely. Just because most of the bullets don't hit you if you are wearing your body armour, it is still better to avoid them in the first place.
And since the entire idea about PCT comes down to taking advantage of the negative feedback loop that exists between estrogen and testosterone - ie. lower your estrogen and you will thereby boost your testosterone production, there really isn't any advantage to using an obsolute compound.
SERM's do still have their place though. They are superior at stopping high estrogen symptoms here and now, where it will often take some time for AI's to reach optimal effect in vivo.