From the site I linked to above...Do Zoom Binoculars Work?:
The idea of being able to scan an area and then zoom in to get more detail on a particular object sounds ideal, but do Zoom binoculars actually work and if so, how well?
The answer is both yes they 'work', but how well depends on a number of factors.
Let me explain, they do work in that you can zoom into an object. However they do not compare to the quality of image that you get through a good fixed magnification binocular.
The view through the variable ones that I have tested tend to be a little fuzzy in comparison to high end fixed magnification binoculars and they often have a much smaller field of view. This often makes them far less suitable for uses like birdwatching or anywhere where you need to be able to quickly lock onto a subject or scan wide open areas.
I think a telling point here is that of all manufacturers of binoculars, not many/any of the real high end, quality brands like Steiner, Kowa, Swarovski or Zeiss have zoom binoculars in their lineups.
Could this because they don't want to risk their reputation on something that is a little gimmicky or is it because they are not progressive enough? Of the companies that do make zoom binoculars, the best are Nikon, Bushnell, Olympus, Pentax and Celestron.