Yes, it wasn't in vanilla, but if you wanted to tank or in some cases DPS you're kinda forced to install an addon if you want it, and the fact that it's not present in vanilla was kinda weird, considering interupts are already in the game at this point. The same can obviously be said about threat meters, but unlike threat meters, enemy cast bars did eventually become an option, in I think 2007. Having the vanilla cast bars available instead of quartz or Natur (both of which thematically have a different style than the vanilla WoW bar) seems like an option that shouldn't be too damaging to the vanilla feel. (I mean, Kick for rogues is practically impossible to use efficiently without addons, which seems really backwards)
Don't wanna make a huge post about it, but would like to know what some good arguments against this are