Some stereo microscopes are zoomable and from personal experience I consider this feature very convenient. Be aware that some refocusing is necessary when operating the zoom knob. Zoomability requires a more lenses and each additional lens will reduce the brilliance and contrast of the image. This is the price of zoomability. There are other factors that determine the image quality as well, such as coating of the lenses. For this reason a better stereo zoom microscope might still produce a better image than cheap fixed magnification stereo microscopes. There are several factors to be considered, and optical quality is only one factors of several which determines the quality of a microscope.
Compound microscopes for routine and laboratory use are never equipped with a zoom function. The loss of image quality becomes much more apparent at the higher magnifications. Zoomable eyepieces do exist, but they will not increase the resolution of the image. It is better to buy an additional 15x eyepiece if you have something to experiment around with. If you have a binocular microscope, then it would be necessary to zoom for each eye separately, which is not possible.