Being unable to see if someone is out of grenades makes it harder for people to share grenades when they are found. We can see each others ammo and players will typically share ammo if another player is lacking ammo, but this is not the case for grenades (which are much rarer to find than ammo).
eg. I will typically go through the entire level where my team will take all grenades before I get a chance but they will notice that I am low in ammo and not take it. (this is frustrating, particularly as Ogryn)