The sad truth is that all of these "margins of errors" are easily covered by multitudes upon multitudes of addons out there. (Eg omen for abovementioned example). In LAN tournament arena play, one is NOT allowed to use addons of any sorts, which puts the focus on the player's skill instead of his gadgets.There are things you can react to in PvE encounters that may lesser your margin for error(such as a Pally BoPing a DPS that got aggro)
That said, most top PvPers ignore addons just because they have to live w/o them in order to practice for LAN arena play.
Name me ONE top end raider who doesn't use addons to make his life easier and I'll take my hat off to both him and you.