1) Discussions of realism in this situation are somewhat of a farce since getting your weapon shot out of your hand is not realistic in the first place.
2) I agree that RO soldiers should have some base unarmed method of attack, even if it is near-suicidal. If you somehow lose your weapon when you're 3 feet away from someone, you're not going to do a 180 and run 10 yards down the hall for cover when he's about to shoot you. Trying to grapple with them seems the most natural and likely option in that situation.
Combat knives are a good idea, but it doesn't help in the situations where you just got your weapon shot out 2 feet away from the enemy, unless you automatically draw your knife the split second your drop your weapon, which would be as unrealistic as the auto-grenade-switch from the mod. Knife/bayonette could still be a usefull option for situations where you lose your weapon, but have time to prepare before you go into close combat (ie you lose your weapon, but hide, then take an enemy by surprise). Also, I suppose in those neck-to-neck situations, being forced to draw your knife at close range may be better than trying to work out the complicated "grapple" system that realistic bare-handed combat would probably require.
But, it would still be cool if when unarmed, you could try to rush the enemy, grab his gun, wrestle him to the ground, etc. It's probably too complicated for RO though.