I would use them exactly as I would a windshirt, for protection from wind and light precip. I think the best use for UL wind pants is when you would wear running style shorts and want the extra protection of long pants for cooler mornings or late evenings, if the weather turns cold, pretection from bugs, and so forth. I think this $20 alternative is better than shredding a $60-$80 pair of Pertex wind pants, or spending that much for very occasional use.
I haven't been a fan of wind pants because pants tend to take more of a beating than tops do. I normally wear nylon zip-offs or soft shell pants. If the weather turns bad in my locale, I would usually be turning to rain pants for more protection.
I would love to see this style offered in a lighter color for sun protection, or bug protection in sunny weather. I think the black color would be too hot in full sun.