My take on this is: you don't need Zeovit to run a successful tank. But you can use it to help run a "really successful" tank. For example, going back to Brad's thread about "not all live rock is created equal", what can you do to promote the ideal bacterial growth to consume nitrates and phosphates? If a bacterial based system can be managed to promote a low-nutrient system, then it can level out the playing field a little. Plus, encouraging the rampant growth of these bacterias to rapidly consume things like nitrate and phosphates and then be skimmed out via the skimmer, allows you to in turn ramp up your feeding, which in turn, has its own benefits (since ordinarily you're limited by the bioload capacity of the tank - so the limits are increased
).