I have both Mordkainen's Tome of Foes and a pre-errataish(correct Ancestral Guardian dice but pre-nerfed Healing Spirit) Xanathars Guide to Everything. However, I never got Volo's Guide to Monsters and my Tasha's Cauldron of Everything Alt cover was stolen two years ago by a package thief the day it was delivered to my apartment complex.
So I pre-ordered the Rules Expansion Gift set knowing full well that I technically have "two outta three(four technically since the new Mord book is basically Tome of Foes+Volos combined together" and the fact that when it comes to using any copy of Xanathars, it will be the single version I already possess as I do not believe in the nerfing of things in already released books.(meaning the Errataed version of Xanathar in this gift set will pretty much be mostly window dressing/coffee table style displaying.)
But I'll finally have an Alternate cover of Tasha's in some format(without paying stupid scalper prices) and that I'll have a number of the 5E races all in one book. Which for me is a win win. Plus outside of not having the Mord, Volo, Xanthar Alternate covers, all my current 5E books have been the Alternative covers.