It depends a lot on the particulars of the trip (e.g., location, season, weather forecast, etc), but yeah, I hike sometimes without any shelter.
Generally, for southern CA hiking in the summer, I'd say you can expect the weather forecast to be pretty reliable and if the forecasts call for sunny clear weather, you can be reasonably sure it will be nice. That said, I'm just over the hill from Ojai and the Sespe area (right on the coast) and I woke up this a.m. to heavy marine layer and drizzle. Depending on how far inland the marine layer crept in overnight, it could be foggy and light drizzle along the Sespe overnight and then burn off early morning. The boundary line of the marine layer seems to hang right around Ojai...
Depending on where in the Sespe you plan to hike, you could always set up sans shelter in one of the many caves or rock overhangs... Lots of nice spots up on Pine Mountain.