Why mark targets?

[Bridgeburner]

because people do not know how to read these

Because those aren't readable without manually clicking off to every target, which interferes with the ability to deliver skills on GCD to your primary target.

Also, your second graphic is actually showing the health bars of the enemies remaining on the field - not the threat. In the pic:

Top Shade Seeker is T; full health, full bar
Middle Shade Seeker is R; partial health gone, partial bar, to scale.
Bottom Shade SEeker is Q; full health, full bar
Clam is S; partial health, partial bar, to scale.

People like marks because many tanks don't Flash regularly enough to keep AOE threat established. No good DPS wants to overrun the tank.