Always packed the rain fly until that one dry week in the Smokies
I used to pack the rain fly for my tent every single trip, even in summer when no rain was forecasted. Last August I did a 4 night trip near Gatlinburg, the forecast showed zero chance of rain the whole time, so I left the fly at home to save weight and let the stars show through the mesh. First night was perfect, second night was perfect, third night a random storm rolled in at 3 AM. I woke up to water dripping on my sleeping bag and spent the rest of the night huddled under a tarp I had in my pack. Cost me a $60 sleeping bag that never dried out right. Have you ever skipped gear because the forecast looked too good, and did it bite you too?
Learned that lesson the hard way a few years back in the Olympics, where it can dump rain out of nowhere even when every app says clear. Now I bring a fly that weighs almost nothing, one of those ultralight ones that packs down to the size of my fist, so I get the stargazing without the risk. If the sky looks iffy at all I'll just clip it on before bed, takes like two minutes even in the dark. Worst case I carry a few extra ounces I never use, which beats losing a night of sleep or a bag. A cheap emergency poncho tucked in the pack works too if you just want a backup. Sleeping dry is worth way more than the tiny weight savings.
So "the forecast showed zero chance of rain" is basically the mountains laughing at you behind your back, huh? A dry week in the Smokies is about as real as a free lunch. That random 3 AM storm did not care about your app, your weight savings, or your plan to see stars through the mesh. It just showed up and took your sleeping bag as a tip. I did the same thing once in the Sierra, left my fly behind to save a pound, and got woken up by rain hitting my face at 2 AM. Now that fly goes in the pack every single time, no matter what the radar says.