In my opinion, ankle extension is largely individual - that is, some people will naturally extend their ankles considerably while lifting and others won't. The latter group should NOT be encouraged to extend, and the former can be allowed to extend IF they maintain adequate speed and balance (which many don't).
The ankles mechanically are great for generating force but not power because of the lever type. So ankle extension is relatively slow and consequently doesn' usually add anything meaningful to the aceleration of the bar. But it does add time to the athlete's extension and elevate the athlete's body, which means no gain in net bar elevation and less time to pull under.
Even athletes who extend the ankles considerably during the snatch tend to do it very little in the clean. So again, bottom line - if the athlete doesn't extend the ankles but cleans/snatches well, leave it alone. If he/she does extend, they better be fast as shit and maintaining their COM over the front of the heel even when on the toes.