Some thread resurrection here, but I've been doing some thinking lately.
Like Eric, I normally wear light-colored softshells as long as the highs are lower than around 65-70 (often in the mountains). Any warmer, and I'm much more comfortable in shorts.
As such, when the weather patterns dictate that I'll usually be hiking in shorts, it'd be nice to have something warmer to wear in the early mornings, evenings and if bad weather moves in.
Typically, I'd just be carrying around softshells in my backpack, but that's a lot of unnecessary weight to be carrying, if the weather for the entire trip appears to have highs that dictate mostly hiking in shorts.
I think this is where wind pants are most useful, as Nigel has already stated. When you're typically preferring to hike in shorts, but need some insulation in the early mornings or late evenings (or unexpected large weather shifts). I'm often in more manageable terrain during these times (off ridgelines/talus fields/etc.), or even in camp, so wind pants durability isn't much of a drawback.
So, this would typically be, extended highs above 70 degrees (80s and 90s included) during the day, but in a mountain or marine climate, where cold winds/rain are quite possible in the early and late hours.
I'd like to experiment again with regular hiking pants used when hiking in these temperatures, but based on past experiences, I'll likely be more comfortable hiking in shorts and carrying the extra 3oz for windpants to prevent chills in the early/late hours, or in unexpected short-duration bad weather.