It's not true that their future forms are hardly ever used as imperatives. They are.
It's just that these sorts of verbs happen to be of a class whose imperatives, as an exception to the rule, are used in common situations. More accurately, it's a "new imperative" rather than the classical imperative, as you can see in some corner cases. It's usually formed by dropping the "t" of the future tense. For example, the plural would be azvu, instead of izvu. Similarly שתקו would be shteku instead of shitku. And in verbs like שמע, which start with a sibilant, you'll actually sometimes hear "tshma". The distinction is also noticeable in nif'al and hitpa'el, where you'll find forms like kanes and stalek instead of hikanes and histalek.
