You don't want to be up on a ridge in a storm, but at/above treeline is fine. An alpine basin (presumably heading towards treeline) is plenty safe in a thunderstorm, as is an alpine ridge/pass in rain before ~noon, or after ~5pm. In these situations, coupled with high winds, a poncho won't work well.
If you don't mind being wet, you can just not wear a poncho until you're below treeline. This works well as long as temperatures are expected to be moderate and you're not going in and out of treeline. If you get stuck up high with really cold rain, even an hour or so can be miserable. So you either need to use your poncho in the alpine wind (which sucks), or use a rain jacket.
I tend to not bring any rain gear on shorter trips when the temperatures will stay moderate and limited rain is forecast, relying on my shelter when I stop. When temperatures get colder, heavy rain is forecasted, or on longer trips, I bring a rain jacket. I used to bring rain pants, but after I started hiking in softshell pants last year, I only used them once (near-freezing rain). I'm going to try a rain skirt this year, which is less weight, less fiddle-factor, and, when layed flat, has many other uses.