That's a pretty dirty workaround. It works well since Piratez is a master mod, but for general mod, that's not a great idea. What about the vanilla grenade? For regular mods, you'd need to redefine its handob too, and then if some other mod has a grenade and didn't bother to make a new handob, suddenly nothing will display. If that makes it into the Dalek mod (such an awesome thing to say!), then someone wanting to play just vanilla+Dalek, or vanilla+Dalek+other mod, needs to deal with that too.
In general, if you don't set a handob for an item, a grenade will be displayed. If you want something else than a grenade, you should define it specifically. It is better regular mod design (where you only change a few things, not for Piratez) to just define an empty frame and reference it in your item declaration if you don't want anything to show.