I've often gone into the Sierra in summer without dedicated rain pants (the solubility of human legs in water is very, very low - grin- ). It may impose some limits on your travel - there are passes I wouldn't do during a storm without rain pants over some insulating layer in 35F, 30 mph, and hail or wet snow falling - which can happen at elevation any month of the year in the Sierra. I'd hole up somewhere lower and take half a rest day, cook some food, veg out on the scenery while waiting for a clear window. If it was just your basic afternoon thunderstorm and I had the batteries along, I might do the pass that evening. But if it was a tropical storm moving through, I might be pinned down for 24-36 hours.
If you don't have the days and months and years in the mountains to distinguish a thunderstorm from a larger weather system, then I'd strongly recommend the extra few ounces for the rain pants. Because of the dead-air space under them and their keeping your base layer drier when you most need it, those few ounces are partially offset by needing less other clothing.
We went in one time during tropical storm "Irene" and got rain every dang day sometimes for most of the day. We came out 8 days later and commented to the ferry boat pilot, "Man, Irene sure went on a long time!". He said, "No, Irene was followed by Julia and Karen." So it can happen and for anyone with less than 50-100 nights of high-Sierra experience, I'd put rain pants in the safety category.
Jennifer, if yours are "kind of heavy" maybe a lighter pair should go on the birthday/anniversary wish list. Valentine's Day is coming up and guys LOVE to be told what to get you.