Backpacking Shell Jackets
A BACKPACKING shell needs to be quite durable and probably won't be the lightest jacket in the store. The reason is that you will have hip belt and shoulder straps pressing down on it - forcing the water through the fabric, and if they move as you walk they will rub the fabric causing wear.
A BACKPACKING jacket may be quite lightweight but have reinforcements where the shoulder straps or hipbelt make contact. The choice may also depend on where you are going - one principle of going Ultralight is to understand the environment you are going into so you can choose the lightest gear for the job. If I'm going for a month long backpack to Yosemite in the summer I now it won't rain very much - even though I'm backpacking I could take a very lightweight jacket because I don't expect to have it on very much.
If I'm heading to northern Norway then I'll expect a lot of rain and I know from experience that a very lightweight jacket may not stand the pace if worn for eight hours every day - I'd take a heavier jacket, either eVent or Gore-Tex. If it's a winter trip to Scotland then it will be heavier again because I could be caught out in a hail storm and my jacket could be my only protection.