Builds in WoW have always, to some degree (and maybe more now than ever) been largely influenced as much by social factors as actual math.
In this case, I think we see a very good example of how certain builds become popular to the point of being the only build that is considered correct. Fire mages particularly excel at these AoE situations with fairly persistent adds. So much so that if you lean into that and specialize your build for it, you can do ridiculous (and fun) burst damage on certain pulls.
From the perspective of high-end mythic+ and mythic raiding players, the only reason to bring a fire mage (or any spec really) is for what they are particularly good for. Fire mages can make certain pulls in mythic+ possible because of the insane damage. It doesn’t matter if they don’t contribute as much on the boss fights or shorter pulls because you’ll get that from someone else - what matters is that they can make a couple big pulls possible. In mythic raiding, people aren’t struggling that hard to kill most bosses from a dps perspective at this point, and they just want to get ridiculous parses and see how far they can push it. So, it makes sense to heavily specialize the build for those moments where you can be the best at something in the game.
If you are a player looking to play at that level and specialize on those moments where fire mage can do silly things, it makes sense to stack up mastery super hard. If you are a player using a fire mage in more balanced situations and aren’t being brought to a group specifically to do that one job of burst aoe damage every 2 minutes… then you’d probably want to use a build more like what we suggest by default. We think that it makes sense for the generic advice to apply to a generic situation. Specialized advice should be labeled as such and the assumptions required to make that advice “optimal” should be stated.
The customize options allow us to specialize builds - we’re working on some updates for shadowlands that can allow us to provide some customized builds pre-canned for people to choose when situations like this arise. I think that would give people the best of both worlds in the optimizer.