I always have my weapon set at the highest possible rate of fire.
There are two distinct advantages to higher concentration of fire.
1) Greater volume means greater suppression. This means in a shoot off between you and an enemy, you are much more likely to produce more suppression if you were in Auto/burst and he was in semi. This will drastically reduce his fire effect, and therefore improve your survivability.
2) Massive advantage in close quarters or when you need to engage at an angle you were not expecting. Surprise attacks on you need to be responded to instinctively, and most often you only have your first shot/burst. Unless you hit them in the head, you most likely will not kill your enemy on semi, and you can only hope your hit has distorted his shot so you can get another one off. On burst or auto, it is much more likely to get multiple hits on an enemy.
When on Auto, you can always control your fire into bursts. If you have trouble doing that, go play CS:S more, where controlling your rate of fire is essential. When on Semi-auto, you have to pray you will not be surprised because you will not have time to switch.