Roger, I agree with you - for ultralight use. Our situation is slightly different from that scenario though, so I'm not convinced just yet unless there are hard numbers somewhere. If there aren't, I may have to just go with rip-stop for this project and application.
This is why:
"If you made a tent out of 4 ounce nylon you could dive into the tent and have your dog do this also without hurting the tent"
Well, that's kind of what our shelter will have to live with sometimes. In our case, it's not a dog but two very energetic preschool children. Normally they are really good with gear, but when everyone is tired or over excited, stuff happens. The same goes for those rare but hectic moments where we pitch in horizontal rain-thunderstorms and the like, where it will be us, the parents, not beeing super careful with the gear.
In addition, we occasionally have very high winds, to the point where walking becomes difficult for an adult.
From experience, I can clearly say that the Ray-Way sil-nylon is up to all that, as long as it is sewn right. So I'm pretty sure 4 ounce nylon is really not needed even for that. But I have a feeling I would prefer to keep the rip stop in the sil-nylon for our application.
We always go light, but we are not on an ultralight mission, and top priority is gear that requires minimum attention, time and energy from us while out there, while providing a high level of reliability even under less than ideal use - because that does happen in real life, especially with children and on trips that go beyond a simple overnighter.
An action that is tough on gear that you could call a "bumbling user error" on a nice sunny day very quickly becomes an action that is extremely hard to avoid under seriously tough conditions, and that's exactly when I can not afford to have a shelter fail on us.
I have the impression that sometimes, people who go ultralight tend to not think about what actually happens when for example their shelter goes to shreds in a "totally unexpected" blizzard. Of course, there are places on earth where conditions are more predictable, and there are those where that is less the case. Still, stuff happens, and I really don't mind whether I carry 12 or 15 kilograms of gear. Each to their own, of course.
On very long trips I found out that I can happily walk with up to 35 kilograms, and that's not for bragging, quite the opposite: I weigh 70 kilograms myself and am neither a super fit athlete nor a couch potatoe - I think most people can carry a lot more than they think. I am in no way debating that less weight is better for many reasons, and even increases safety on trips as well - that's absolutely correct! But there is a limit to that, and if you go below that, the problems that that can cause in those rare but very tough situations are not worth saving yet another few hundred grams.
Just something to think about, not telling anyone how to go by any means, and I'm very thankful for the ultralight movement because it is continuously contributing to making gear and gear systems better.
Now, back to choosing a fabric... :-)