Although I will probably never use the stink or smoke bomb in battle, they have been very valuable for leveling, and if I were to level grenades again on another character, I would put a point on each of them again, for the same reason. Not only do they afford you more exp points per kill, but if you use both, you also have more of a supply of ammo (which can save time if you're sailing back and forth somewhere to work them).
They can be more difficult to use, though, than the other grenades (maybe because their area of effect is wider?), and it's easy to damage or blind yourself with them. They become easier to use with practice, but to minimize any damage to my character, I usually toss a few of either or both of those, and then switch to fire or basic grenade to finish off the target.
Where ever you choose to put your skill points, good luck! I like them better than I used to, but seriously doubt grenades will ever become one of my favorites.
