A friend of mine was in Nam and said that he prefered frag grenades to his rifle because once a shot rings out everyone hits the ground and you end up shooting over someone (plus as you mention the bullets deflect in heavy foilage). Those frags are a lot more effective than small arms. He also said that you can take cover from bullets and feel sort of safe even when bullets hit close or zip by but nearby explosions are unnerving.
Just an aside he taped one grenade to him so he wouldn't throw his last one in the heat of things. He wanted to save one for himself just in case - he wasn't going to be taken alive if things got sticky. He spent all his missions behind the lines doing recon and ambushing and such. He had a camera with him and took some amazing photos. He smuggled the film out and developed it all when he got home. Those pics could get guys in real trouble if they ever got out - documenting things that aren't suppose to happen, etc.