I currently use a footjoy hydrolite short sleeve sport windshirt and a Columbia Rebel Roamer Rain Pant. I also carry the Gustbuster umbrella.
Playing in the midwest means that spot storms pop up on a regular basis. Last year we were walking a nice little nine hole track and storm came rolling in as we were finishing hole 3 (furthest hole from the clubhouse). It started with those big rain drops, you know the ones that indicate you are about to get a thorough wash-down, then within the next two minutes the wind picked up to the point that an umbrella was useless. Behind the storm front was a constant wind and light rain so we marched through the next 6 holes. My old umbrella was dislocated and I have a strong mental note to always carry some rain gear in my bag.
I play in all conditions and I like to use the rain gear when it is raining or just wet conditions; nice to be able to grab a hose and spray off the pants then dry with a towel after a round of golf. Do not want to ruin any nice golf clothing because of sloppy conditions on the course.