When I was first lightening up my backpack load I was doing research into various types of gear. At the time the idea of water resistant clothing for rain (rather than water-proof) was pretty hot. I read several articles and decided I'd give it a try.
So, I ordered a wind-shirt from Montane (a European company). It weighed 4 ounces. I quickly discovered that a water-resistant jacket was insufficient for my needs. A couple of years later I was backpacking with my parents in bad black-fly country. I was carrying non-breathable waterproof rain-pants, and wearing shorts. Our legs were literally dripping blood from 50 or so places each due to the black-flies, and I finally gave in and put on my rain-pants as bug protection. That evening I took them off and my whole lower body was soaked from sweat, and I had pretty bad chaffing the rest of the trip. I thought about the windshirt and realized that similar pants might fit the bill perfectly.
(I'm on the right, in the black wind-pants and black tee-shirt)
Well, I've had the 4 ounce pertex wind-pants for maybe 6 years now. They've seen over 2000 miles of trail, as well as use as x-country ski pants, snowboard pants, rain-pants, anti-bug pants, sun-pants, and laundry-pants. On one trip we bushwhacked through wild roses and blackberries for 4 days. I've also bushwhacked through manzanita and live oak wearing them. I wore them for a couple of years as my only long pants (often over warm tights). I ran to work 2 miles every day (often through the snow) for an entire winter. I used them as my long-pants (wind/rain/warm/etc.) for the whole AT. I've slid down rocks on my butt in them. Basically, it's hard to come up with something I haven't done in them (short of going to a cocktail party).
After all that wear, they still only have one hole. On the AT I knocked over a burning alcohol stove on my leg (spilling burning alcohol all over), and it melted through before I was able to put out the alcohol. This hasn't significantly affected the pants' utility, and I'm still using them.
So, anyone who claims thin Pertex pants aren't sufficiently tough, is going on instinct, and hasn't really tested them. They have a downside of course, they are not waterproof. Basically, they do everything rain-pants do, and do it better, except for stop water :P.
BTW, to be clear I would not recommend a thin Pertex pant for bushwhacking through thorn-bushes. They'll tend to stop you from getting huge gashes down your legs, but you will get scratched up a bit. Still, for 4 ounces what do you expect? On many trips I wouldn't have carried something else, but I had these because they're so tiny and light.
I also strongly strongly dis-recommend use of these pants for activities like snow-caving. Cross-country skiing is one thing, where you fall in the snow and get back up. For prolonged direct snow contact I've discovered I want something really actually waterproof, or really thick wool. In all my years carrying them, snow-caving is the only time I wished I had waterproof pants instead (though that time was pretty bad).
Long and short of it. These pants are awesome. They are some of the toughest gear I've ever owned despite being 4 ounces (I know, hard to believe right?). They're great unless you really need something waterproof, and in my experience that's relatively rare. Don't ever let someone tell you rain-pants are tougher them wind-pants, pick between the two based on your needs for water-proofness, not durability.
One final note: Going up in the Sierra on edge-seasons and winter has changed my use-case a lot. I just got a pair of water-proof breathable rain-pants. We'll see how that works out - Stay tuned :P.