It's very unrealistic to expect close to 100% time on target in a combat system like the one TESO has. Time on target will vary wildly based on many factors. In wow, time on target is much more predictable.
For example. If someone does 10 average damage per hit, attacks 1 time per second and has 100% uptime, then they'd do 10 dps over a period of infinity. If you cut the time on target by 50% though, you cut the dps by 50%. Balancing around time on target is inherently flawed in a game that promotes blocking and dodging as core mechanics, and you're inevitably penalized for doing anything but balls to the wall dps, which is not how the game is intended to be played.