Using +/- for light

I've got an S 2IS and have tried using the +1 or +2 for low light areas in trees or shrubs for bird shots. It appears to be helping with the clarity especially when I'm shooting at 3.5 aperature which is fully open. Will I be getting more noise or are there other issues when using the +/- feature?

I am not familiar with the S 2IS. However, I think you are referring to exposure compensation. I have found that +1 with a clear sky is helpful with my camera (a 30D). I have had to use as much as +2 on a bright overcast day. Exposure compensation is wonderful for bringing out detail in your subject against a bright background. This is helpful in a lot of other situations too. If your camera is set to aperture priority, compensating + will decrease your speed. Could be a disaster if you are shooting moving subjects. If you underexpose your subject and try to brighten it up in post processing you will end up with a noisy image. If you are going to miss the exposure, it is best to miss a little on the hot or + side so you will be darkening it a little in post processing.
All this said, nothing beats an incident meter.