Montbell makes some windshirts that are really light and affordable ($75-90 and often on sale). I got the Montbell UL Wind Jkt on eBay for $35 a month or two ago and these are often listed but there's none right now.
It's really a question of how much durability do you want in your clothing system and where do you want it? Montbell makes windshirts with 7D nylon (same fabric as the ex-light) and 15D nylon and they are very light at 2-3oz. These shirts work well as a windshirt (ie. avoiding wind chill) and they will transfer a lot of the wear off of your Ex-light onto the windshirt, but they won't hold up that well to sparks or bushwacking. A big spark can pass through these and through your down jacket, so you need to decide how much protection you want. You need a windshirt in the 5-6oz range to really give you a second to brush off sparks.
The traditional backpacker would likely bring a heavy durable insulating jacket, heavy durable rain jacket and a heavy durable windshirt (and more). By using either the windshirt or rain jacket over the down jacket, you avoid the need for a more durable windshirt. This is a simple UL technique to save weight with no major downsides. If you want to take things a step further you can either reduce the durability of one/both of these garments or leave one or both behind. Either of these decisions comes with a more significant compromise and responsibility.
Around camp, either of these garments will do fine to protect the down layer (except from maybe large fire sparks). I prefer to use the windshirt as they are cheaper to replace. My wife put a few holes in her rain jacket when wearing it over her down jkt around a fire.
I wouldn't carry a wind shirt just to protect the ex-light. I carry a windshirt for other reasons, and then I wear it over the ex-light around camp because it's the best choice (less critical and expensive than the rain jkt). If you don't carry a windshirt anyways, then just use the rain jkt over the ex-light. I carry a windshirt because it's the most comfortable thing to hike in when it's chilly out. I find myself wearing it all the time, rather than bringing a heavier hiking shirt. A light hiking t-shirt (2.5oz) + light windshirt (2.5oz) is lighter and more versatile than a heaver long sleeve hiking shirt.
A similar benefit exists with carrying the windshirt and the ex-light (versus carrying the Atom LT). You have 2 separate layers so you can wear just the windshirt if it's a bit chilly, or both if it's colder. You can also replace the windshirt when it wears out and your ex-light will still be in great shape. Two separate layers will also be easier to dry than one combined layer.
FWIW, My summer upper body clothing setup is the Ex-Light Vest (3.6oz), Montbell UL Windshirt (2.7oz), Marmot Essence Rain Jacket (6.2oz) and a really light synthetic hiking t-shirt (2.5oz).
Sorry about the long winded post.
