I think the versatility of the abilities will really depend on how Bungie meant each of the slots. So if the "grenade slot" only has abilities that are literal grenades to throw, then sure, that wouldn't be too interesting. But if instead the abilities are simply projectile abilities in that slot, that might be more variable. Plus, in the gameinformer interview, it mentions being able to tweak each ability, so you could have a "grenade" ability that did a lot of damage, or one that instead left a damaging area in its wake that remained for a while. Focus-dependent passive abilities were also mentioned, but I'm not sure how much those will affect the other abilities. Lastly, if you go back and watch the Devil Walker footage, you'll notice a "grenade" thrown by a Titan that seems to immobilize the tank, or provide some sort of beacon.(I didn't find this, I believe either destinyupdates or another youtuber did. I just remembered it) So, to sum it all up, we really won't know til the beta comes. I feel like, most likely, Bungie will have used the three slot method as well as they could, so we will probably have a good amount of versatility, but it won't be quite as diverse as allowing more ability slots or allowing any ability to go in any slot.