Matt Laur , Feb 22, 2010; 08:25 a.m.
It's possible that you're confusing the range of the zoom lens (meaning, the difference between its widest focal length and its longest focal length) with the magnifcation of the zoom lens. The 100-400mm lens provides significant magnification ... but its range of magnification only goes from "quite magnified" to "very magnified" - as opposed to the "super zoom" stylle lens that might go from "quite wide angle" to "quite magnified" in an attempt to be more generally useful. But that huge range of focal lengths always comes with optical compromises. When you want high quality lenses that don't introduce artifacts, or softness, or small apertures, etc., you end up paying a lot more money for what can appear to be less versatility.
The difference is that the more specialized lens will produce far better results than the do-it-all lens with a wider range of focal lengths. Whether this makes sense for your type of shooting is a personal judgement call.