it's all about control... if you feel that you have sufficient control using A or S mode, go with it... just make sure what you're metering (spot, centre-weighted, etc), and play with +/- EV, if that works for you (and your camera, depending on how easy it is to +/- EV).
with digital, you can shoot, check, adjust, and shoot again... but some situations might not present themselves again, as with the case with some event-type or photo journalist-type photography. it can all go wrong with any of M, A, S mode... but if you know what you're doing using, let the various modes work for you, and not be a slave to the mode that everyone says that you should use.
that been said, the modes provide specific controls over the Aperture and Shutter speed, and have distinct advantages in certain types of shoots, e.g. when you want to control the DoF or exposure time.
on a separate, but related issue... shoot RAW!!