I have noticed that gear lists typically include a wind shirt and rain gear. Being a canoeist transitioning into backpacking, I have always used my rain jacket to stop the wind. Why would I want to pack both?

Wind shirts usually let sweat through better than rain jackets. Also, if the wheather is good in summer, you can go without rain gear and just bring a wind jacket.

The reason that you might use a wind shirt instead of a rain jacket is generally speaking, the wind shirt will be more breathable, so for hiking, it often preforms better than a rain jacket. I tend to sweat more when hiking hard in my rain jacket than I do in my wind shirt. If you were using it for camp use only then there would be no advantage.

This is a hot topic so I am sure you will get some people who love and hate wind shirts.

Simply because they are more comfortable and weigh next to nothing.

Sun, wind and bug protection for just 4oz.

And not only do they weigh mere ounces (the MontBell weighs around 2.5 oz) but you can give them a DWR treatment and then they'll shed summer showers while still being able to breathe. They won't handle a hard, driving rain though; I resort to my poncho for that, which also doubles as my shelter.

In addition to the wind resistance, I also find a windshirt adds a fair bit of warmth to my layering system in colder weather. I often wear one over a merino top, and under an down jacket. I think the windshirt helps trap warm air close to my skin, preventing convective heat loss from air circulation inside my clothing.

Plus, if you spend much time hiking UP exposed ridges in moderate weather, windshirts are essential. Raingear is too hot, and no-protection is too cool. Buy one, and I bet it becomes one of your most-used, most-carried pieces of gear.

I think the ID Pertex jacket is sweet, or the Patagonia Houdini. Or you could make you're own with a thruhiker.com kit for only about $40.00!

I bought a wind shirt for just the reason Scott suggests - a lightweight warming layer when touring or downhill skiing. I've been using down instead of fleece for insulation and the wind shirt (mine is made of EPIC) adds some necessary protection against wetting out from spindrift and falls. A lighter alternative to a softshell. Not so keen on their use in summer, though the EPIC makes it a workable substitute for a rain shell when it's ground fog or a light rain and not a downpour. Strictly for wind protection I think one is surplus - a vented shell doesn't cause me to overheat and I need to take the shell with me anyway in case it rains.

I left my wind shirt home when I went to the Wind Rivers last summer, because when it is windy there it's usually cold enough that even a non-breathable rain jacket isn't a problem. However, I discovered that on warm, windless days, the horseflies regarded my permethrin-sprayed shirt as just so much appetizer and chewed away on me at every rest stop. Next time, that wind shirt goes with me, unless I can find a comparably light suit of medieval armor!