Anyone who cares about whose bar is the longest is already measuring on their own screen. Not only is the reporter almost always on the top (and conveniently never reports when s/he is below), but displaying the damage done for a fight to the same raid who's on the meter is just pure epeen spill. Asking for a damage meter is just laziness (or, in rare cases, a really crappy computer paired with a log-intensive fight).
Let's not forget that problem of boiling a player down to a single number. All three roles of the holy triad have a complex set of abilities for every encounter.