Of the varied people I know that play RPGs, variance in numbers output has never been considered an integral part of an RPG. With respect to WoW, I generally see people wanting less variance in player output by control being more directly related to player input, not some RNG factor behind the scenes.
Also, averaging out to be negligible does not mean the effect won't be negligible. Blue post states "most non-periodic spells"... alright, which spells? What's the impact on specs that are mostly DoT/HoT oriented versus those with mostly direct damage abilities? What if you're a spec that relies on infrequent, large-damage direct spells? Over what duration of time is this supposed to be averaged to 'negligible'? Is there "bad luck protection" so you don't get constantly positive or negative background spell amplification? Why not apply this to all spells instead of just direct damage? To my first statement: who even asked for this and was it a problem having static values versus behind-the-scenes random damage variance? If the current system isn't broken or an issue, why 'fix' it?
I keep hearing Blizz repeat the mantra of making SL more like an RPG. However, I feel like Blizz keeps adding elements typical of single-player RPGs that don't necessarily translate well to an MMO. I'd rather have Blizz making game decisions based upon how fun/entertaining they are. When I think fun with a game, forgive me for not immediately thinking random damage values variance in the background. If I'm playing D&D, the variance in my damage output is fun because I feel like I am in direct control of it by rolling my dice... and that behavior doesn't translate well to WoW's combat system.