As stated before marines are not supposed to hold off the mob completely by themselves. However the only time they get overpowered by zombies is when a player is nearby, which means that it's the players fault if they're overpowered. Giving them an ability for crowd control would increase the safety of the camp zones => more campers= less people doing objectives.

If a marine spawn is gang banged by NPC zombies YOU will have to clean it or wait until the zombies die by themselves. Don't expect the military to do everything difficult for you.

BB isn't about realism, so marines might have grenades in reality (well, I'm pretty sure they have) but not in BB, for game balance reasons.

The useless grenade animation doesn't really waste any big amount of time IMO and the dev team was wether unable or didn't care to fix it. This might get a fix in the future patches, but since it's mostly cosmetic it won't have a high priority.

LOL! Just give the marines the nades, they do crap damage to non-player zombies and they only throw them rarely. ...Or better yet, have them throw soda cans!!!

Thank you Nekrotzar, that is why I asked about reasons, considering they get refuted and people act like they weren't...