The bars are either filling or depleting, and for the sake of simple arguments, lets assume they are filling. Their length is typically the same, no matter the whole time to complete. This leads to displays like the following:
OOOOOOOOO_________ OOOO______________Which one is going to finish first? You will not know without reading the label or watching the speed of the movement, which defies the whole idea of a bar as a representation of time... Also, if the bars sort themselves, so that the top ones finish faster, then the bars reshuffle occasionally, which is distacting.
Moreover, the timers are often scattered and harder to compare, since they come from different mobs. (Will I have to refresh scorch before Malygos heads into Vortex? The answer takes more than a single look)
A simple design sketch
The basic idea of this design is a timeline from left to right, future events are to the right. As time progresses, the bars move tho the left, while retaining their length (Think of Guitar Hero ) This layout would emphathize the relationships of the cooldowns/events/buffs to each other, in order to help the player plan her next actions. Personal Events on the top, Boss events to the bottom and events that are to far away are indicated to the right.
A basic addon could provide the functionality, with modules for different types of events (buffs, swing timer, pvp, bosses, dk runes, dps rotations ... )
Can't we do better?
Since bar mods have a big impact on wow visual style and player performance, I think we should look for new ideas here. I outlined one above and I am certain you will have some as well. /discuss