UV filters are designed to reduce haze when used with film but, as I understand it, have no effect on digital sensors and the main benefit is to protect the front element of the lens. This does add another layer of glass which will reduce image quality, but provided a good quality multi-coated filter is used the effect is neglible.
Back in the day, Skylight filters were also popular as they both reduced haze and warmed the image by adding a small amount of magenta. Used with a digital sensor this can have a slight effect on white balance, although AWB would cancel it out.
The coating on a modern lens is very tough, so in most cases a filter is not necessary, but they can be useful as a form of insurance in helping to protect against impact damage. It's easier to replace a filter than a front element. If I were shooting in conditions where there was a lot of sea spray, I would definitely use a filter to protect the lens from the effects of salt corrosion.
As to my standard practice, a few of my lenses have UV and clear protection filters, but most don't. Which makes me undecided, I guess.
