I hiked the first/dry part of the PCT in 2008 with just a poncho tarp as both shelter and rain gear, leaving Campo in late April (right after the ADZPCTKO event). I brought a very light (water resistant, not water proof) bivy, but only crawled into it a couple of times; I wouldn't bring it again.
That said, I had essentially no rain in those first 700 miles. It's possible, of course, to get hard rain in there, so you do need to be able to manage. But it should be a relatively rare event I suspect.
I think there were a couple of nights in the first 700 miles where bugs were a little bothersome; I used the bivy for that, but could certainly get by for a night or two with just a head net.
As you said, I pretty much cowboy camped (no shelter) every night, with the poncho handy on evenings that looked even the least bit sketchy.
In any event, so long as you can deal with the possibility of hard rain and possibly even some snow, I'd err on the side of lighter weight and minimal required shelter/gear.