I turned to EI long ago basically I DIDN'T want to test. Testing gets old really quick, plus your dealing with inaccurate readings many times.
When you dose EI your estimating the needs of the plants. By changing the water your keeping things within a certain range. There's nothing really in EI dosing that is going to hurt your fish unless the amount your using is way off the chart. The other side to a water change is keeping the organic levels low in the tank. The are many way to dos this, but nothing is as free and easy as a water change to keep things within range. Too much organic content will cause algae problems in most setups