Yes, you could do that, if your purpose was to match whether a particular image was an eye having the same proportions as the template.
Templates are normally used to match particular areas of an image, under the hypothesis that the pattern might exist in the image but not necessarily in the same place relative to other parts of the image. Irises are not always in the same place or same proportions, fundus are not always in the same place, and so on. Templates permit matching to focus on a particular part.
When you use an entire eye template, you can match something that is close enough to your eye template, but you might have difficulty with the details being different sizes or shapes. You could always try it and see if it works well enough for your purposes.
