Good, Glad you said that, they can't be client only, as no bot program would ever work in pvp.
When you cast a spell a human player reacts by reflecting/cloaking/amsing as they visually see that cast time/animation.
A bot uses code, it's not sat in front of your monitor watching tidy plates! Therefore that information must hit the server the instant you begin to cast ANY spell otherwise the bot would never react to any spells.
This is also how the addon GladiatorLOS works by interrogating the server, when a opponent mages casts poly on me I instantly get "poly" (lag dependant) in my ear, so they can't happen client side only.
This is the crux of the argument, because it happens at the server the data is there to allow a bot program to reflect.
If your going to comeback with, "we don't know for sure how the log behaves" then lets agree to disagree, however I've given FOUR very strong examples of why the log should behave as I suggest, and you've given because I think it's possibly another way but no good examples to back it up, we've reached an empass.