Depends on temperatures. Most of my trips are 4 days or less.
Running shorts (w/liner - no underwear) and I carry rainpants if the lows are around 40-50 degrees.
When lows get below 40 degrees, I wear running shorts (w/liner) as underwear, and a pair of very lightweight softshell pants (OR Ferrosi). I can slip them off over my shoes, so they're faster to switch into shorts than zip-offs. But, I rarely need to do this, as they have ankle cinch cords and can be rolled up into a pair of shorts w/o coming unrolled. Similar pants would be Patagonia Simple Guides, and Arc'Teryx Gamma LT pants. I'm pretty sure both are slightly warmer, and they also don't quite have the features, fit, or color that I wanted. YMMV. In really rainy weather, I still bring rain pants, but since Colorado rarely has sustained rain, they usually stay home. I'm going to experiment with using a rain skirt this year, instead of rain pants for sustained rainy weather.
In even colder conditions, I switch out the running shorts for long johns, but now we're talking about winter-like conditions.
running shorts with liner: underwear is unnecessary and extra weight, running shorts are lightweight, great for hot weather, and double as shorts/underwear when wearing long pants
rain pants: I need them for camp warmth when only bringing shorts. While others may not need extra warmth in the morning, I like morning sunrise and early light photography. They also are nice to have if we get some cold rain.
softshell pants: I used to use nylon supplex zip-off pants (REI Saharas) and I find softshells to be a huge improvement in several ways: 1) Athletic fitting so it doesn't billow in wind, but stretchy so it's not restrictive. 2) Warmer in wind and rain, does a great job at repelling our short rainstorms, and just as cool in hot weather. 3) More durable and protective when off trail in willows and scrambling on rock and talus.
rain skirt: for ~1oz, these should replace rain pants when I need rain protection over my softshells to keep my thighs warm in sustained cold rain.