Unless you play druid or monk, or rerolled at some point after NH, or simply want to play an alt without playing a shit version of their spec. Things like the Arms ring, Fire belt/bracers, Feral Soul, WW boots, Demo shoulders and many others have a huge impact on how the spec plays, in nearly all cases making the spec significantly more fun/interesting to play, meaning alts/rerolls without those, beyond being a lot weaker, are also less fun to play. That is a problem, regardless of whether or not mains have "beaten" the system(which does not make the system good, it just means you've beaten the system and have all the legendaries).
And he's absolutely right about titanforging devaluing mythic loot, claiming anything else would be silly. You can quite easily get to 935-940 ilevel just doing heroic, which means a lot of mythic raid drops will go to waste if they aren't also WF/TF. Disenchanting multiple items on first kills is not fun, and WF/TF makes that a very common occurence, especially for slower progressing guilds.
This is coming from somebody who's fine with spending a lot of time on WoW, but it's not fun to have so many rewards be completely random. If I'm going to spend 5-10 hours a day on WoW, I want to know that time is going to pay off, and in Legion that is rarely the case, because of things like legendaries being complete RNG.