I'm going to have to agree with the feeling that a windshirt is a luxury. It is by no means, an essential piece of gear the same way a rain jacket is: it doesn't protect you fully against rain, nor wind, but on the other hand, paired with a hard shell, it lets you fine tune your exposure to the elements and increase your comfort level dramatically. You can certainly make do without it, but the amount of comfort it adds on an ounce-by-ounce basis is incredible.
I've worn nothing more than an R1 hoody & Houdini from 20º all the way to 5º for backpacking and snowshoeing. It lets me move at a normal pace without overheating or sweating, which in turn means I need to carry and wear less insulation for when I slow down or stop. When I heat up or cool down throughout the day I can simply regulate my temperature by sliding my sleeves up and down, opening the front zipper, or wearing the hood.
In warmer months like spring and fall I'll wear the Houdini over a lightweight, long-sleeved base layer and I can continue to regulate my temperature in a similar manner. I'll even carry the Houdini into the summer, for cool nights.
More importantly for me is how it fits into a typical clothing layering system, which has radically changed the way I layer. When I used to use a hard shell as a light additional insulation layer, I would wear it and sweat in it, knowing that being waterproof, it would dry quickly when I took it off.
If I got cold, I would have to take off the hard shell, put on a mid layer, and then put the hard shell back over it. With the Houdini, I can simply slide a mid layer over it for warmth, whereas doing the same with a hard shell is far less viable due to the stiffer, less breathable material. My hard shell is then relegated for when precipitation is too much for the wind shell to handle, or if I'm above treeline or in an exposed, windy area in subfreezing conditions.
The other great thing about a hooded wind shell is, unlike just about any other material people wear in the backcountry, snow and water slips right off. If you duck under a tree branch only to dump snow or water on you, it will fall right off your windshell instead of falling down your neck or soaking into your layers.