I love tbe XE-1 and the 18-55 zoom. It has become my standard "1 lens system" for photos in non raining weather to do family, travel photography, etc.
That said I have recently bought the 18,35 and 60 on the used market and find them to alter my photographic style into a more creative mode. With them I tend to take more artsy pictures focusing (excuse the pun) more on the composition than just capturing a family photo. This is much the way I tend to use my Leica M's. In fact since I got the 35, I have not picked up my M9.
As for IQ, I do not think you will see a difference in IQ at f/8 (the 18-55 is really very good even wide open). That said, the ability to shoot the 35 wide open and get fantastic DOF control and gorgeous bokeh is simply worth it. The overall IQ is phenomenal especially when the price of the lens is factored in. The 60 is also superb but a bit finicky on focusing, while the 18 (while not as good as the other 2 makes for a great pocketable XE-1.
There are manu good reasons to purchase the 35 but NOT simply for better IQ at f8
Hope this helps,